feat: add context meter and file-based system prompts

Two additions to the chat window.

A context meter in the top bar shows how much of the active model's window
the next request will occupy: system prompt, prior turns, attachments and
the current draft. The router does not report a usable context size in
router mode (/props returns n_ctx 0), so the limit comes from ctx-size in
presets.ini. Streaming requests now ask for usage statistics, which makes
the figure exact after each reply at no extra round trip; before that it is
an estimate marked with a leading ~.

The estimate reads low on models with a reasoning budget, since thinking
tokens cannot be known before the reply arrives. The tooltip says so.

System prompts are markdown files in the prompts/ directory beside the
config, one per file, so they can be edited in an editor and kept in
version control. default.md is global, any other file is a named preset
that replaces it rather than adding to it, and a conversation may instead
carry one-off text or opt out entirely. The choice is stored per session so
reopening a chat restores the prompt it was built with.

Prompt names are reduced to safe filenames, so a name like ../../etc/passwd
cannot write outside the prompts directory, and a name that reduces to
nothing is rejected rather than creating a dotfile.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

1 files changed, 70 insertions, 3 deletions

diff --git a/README.md b/README.md
index 7c05beb..fdcca99 100644
--- a/README.md
+++ b/README.md
@@ -25,6 +25,14 @@ persistent process and toggles like a scratchpad from a Hyprland keybind.
   thinking separately from its answer. It shows as a one-line `▸ thinking`
   summary above the reply, and clicking that expands it. Each reply collapses
   independently and the state is remembered per bubble.
+- **Context meter** in the top bar showing how much of the model's context
+  window the next request will use. It estimates while you type and becomes
+  exact after each reply, turning amber at 75% and red at 90%.
+- **System prompts** kept as markdown files in `~/.config/llamachat/prompts/`.
+  A global `default.md` applies to new chats, named presets replace it, and
+  any conversation can take a one-off custom prompt or none at all. The
+  choice is stored per session, so reopening an old chat restores the prompt
+  it was built with.
 - **Tray icon** for show/hide/quit, hosted by waybar's tray module.
 
 Not in this version: RAG or embedding search over history, multi-user
@@ -118,6 +126,10 @@ db = ""
 default_model = ""
 request_timeout = 300
 
+# System prompt for new conversations: a preset name from prompts/,
+# "none" for no system prompt, or empty for the global default.md.
+default_prompt = ""
+
 attach_ctx_fraction = 0.5
 chars_per_token = 3.5
 ```
@@ -252,7 +264,8 @@ there is no second thread.
 ### Database schema
 
 ```sql
-sessions    (id, mode, title, model, created_at, updated_at)
+sessions    (id, mode, title, model, prompt_name, prompt_custom,
+             created_at, updated_at)
 messages    (id, session_id, role, content, reasoning, created_at)
 attachments (id, message_id, path, kind, mime, size, sha256, thumb, truncated)
 messages_fts -- FTS5 external-content table over messages.content
@@ -265,8 +278,16 @@ finishes.
 `reasoning` holds the model's thinking, kept in its own column for two
 reasons: the FTS index covers `content` only, so thinking text cannot bury
 real search hits, and only `content` is replayed when continuing a
-conversation, which is what these APIs expect. Databases created before this
-column existed gain it automatically on open.
+conversation, which is what these APIs expect.
+
+`prompt_name` records which system prompt a conversation was built with, and
+`prompt_custom` holds the text when that prompt is a one-off rather than a
+file. Storing the name rather than the resolved text means editing a preset
+affects the conversations that use it, which is usually what you want; a
+conversation that needed frozen wording should use a custom prompt.
+
+Databases created before any of these columns existed gain them
+automatically on open.
 
 Attachment *content* lives inline in `messages.content`, since that is what
 was actually sent to the model. The `attachments` table records provenance:
@@ -278,6 +299,52 @@ is gone. Full image bytes are not stored.
 Search input is tokenised and quoted before it reaches FTS5, so punctuation
 that would otherwise be read as query syntax cannot cause an error.
 
+### System prompts
+
+Prompts are markdown files in `~/.config/llamachat/prompts/`, one per file,
+created on first run:
+
+```
+prompts/
+  default.md      the global prompt, used unless something overrides it
+  coding.md       a named preset
+  terse.md        another
+```
+
+They are plain files on purpose, so they can be edited in an editor, diffed,
+and kept in version control. The `…` button beside the picker opens a dialog
+that does the same thing from inside the app.
+
+The picker offers every file plus two extras. A named preset **replaces** the
+global prompt rather than being appended to it. `custom for this chat…` takes
+one-off text stored with that conversation, and `no system prompt` sends none
+at all.
+
+`default_prompt` in `config.toml` chooses what new conversations start with:
+a preset name, `"none"`, or empty for `default.md`.
+
+The choice is recorded per session, so reopening a chat restores the prompt
+it was built with rather than silently applying whatever is selected now.
+
+### The context meter
+
+The bar in the top bar shows how much of the active model's context window
+the next request will occupy: system prompt, prior turns, attachments, and
+whatever is currently in the input box.
+
+While you type it is an estimate derived from `chars_per_token`, shown with a
+leading `~`. When a reply finishes, the server reports the real token counts
+in the stream and the meter switches to those, dropping the `~`.
+
+Expect the estimate to read low on models with a reasoning budget. Thinking
+tokens count against the context but cannot be known before the reply
+arrives, so a model that thinks for 2000 tokens will jump well past the
+estimate once it answers. The exact figure after each turn is the one to
+trust.
+
+The limit comes from `ctx-size` in `presets.ini`. Models the router offers
+but that file does not describe show no limit.
+
 ### Markdown rendering
 
 Replies are parsed by `QTextDocument.setMarkdown()`, so there is no markdown