feat: account balance --at for historical balance (v0.3.2)

Reconciliation needed an account's balance as of a past date; the command
only reported the current balance, forcing hand-written sum scripts.

--at YYYY-MM-DD re-fetches the account with Firefly's ?date= param, which
recomputes current_balance as of that date (end of day, no client math).
Without --at the current-balance path is unchanged (no extra API call).

Verified live read-only against real data: Mediolanum current 183.23 vs
--at 2026-06-10 164.38, boundary probe gives distinct daily balances.

PATCH: new optional flag, contract unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 3 insertions, 1 deletions

diff --git a/SKILL.md b/SKILL.md
index 98b9472..672c97e 100644
--- a/SKILL.md
+++ b/SKILL.md
@@ -52,7 +52,7 @@ If `firefly` is not on PATH, run from the repo with `python -m firefly_cli ...`
 firefly auth test                          verify connectivity and token
 firefly account list [--type asset|expense|revenue|liability|...]
 firefly account get <name|id>
-firefly account balance <name|id>
+firefly account balance <name|id> [--at YYYY-MM-DD]
 firefly account create <name> --type asset|expense|revenue
         [--opening-balance N] [--currency CODE]
 firefly tx add <amount> --from <acct> --to <acct>
@@ -105,6 +105,8 @@ automatically. Unlike categories/tags, accounts are NOT auto-created by
 **Check a balance:**
 ```bash
 firefly account balance test01            # -> {"id","name","current_balance"}
+firefly account balance test01 --at 2026-05-31  # historical: balance as of that date
+                                                # -> adds "date"; useful for reconciliation
 ```
 
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