feat: tx add --skip-dupes for idempotent imports (v0.3.4)

Re-running an import (a retried or double-read batch) created phantom
duplicate transactions and drifted balances, with no signal.

--skip-dupes searches for an existing tx with the same amount + date +
source + destination before writing; on a match it emits
{"skipped": "duplicate", "matched_id": "<id>"} and exits 0 without writing.
Off by default (one extra search per add only when set). dry-run wins over
skip-dupes: a preview never triggers the search. Match is amount+date+
accounts, not description, so genuinely distinct same-value same-day
transfers between the same accounts look like duplicates; documented.

Also document a Firefly quirk in SKILL.md: a missing/deleted transaction id
returns 401 (not 404) on tx get/edit/delete, so a 401 after delete confirms
the record is gone rather than signalling an auth failure.

Verified live on the test instance: create -> identical --skip-dupes skips
with matched_id and no write, --dry-run --skip-dupes previews without
searching, no-match --skip-dupes writes; records cleaned up after.

PATCH: new optional flag, contract unchanged.

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

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