fix: tolerate unreadable prior snapshot in export

[tanem]

Summary

• export reads the existing snapshot before fetching (to carry account data forward and to decide whether to keep the previous file when all accounts fail). It only caught FileNotFoundError, so an existing-but-undecodable prior — most commonly after the encryption passphrase was rotated via set-encryption-passphrase — raised an uncaught ValueError and crashed every subsequent run until the file was manually deleted.
• Fix (cli.py): catch ValueError alongside FileNotFoundError at the prior-read call site. Log a warning naming the file and cause, treat the prior as absent, and regenerate. snapshot.read() stays strict — resilience lives at the call site, which knows the prior is only an optimisation.
• An unreadable prior leaves prior_by_login empty, so failed accounts get empty carry-forward and the all-fail "keep previous" guard is bypassed (the undecodable file is overwritten with a readable all-error snapshot).
• Two new CLI tests; CLAUDE.md gotcha added. No README or schema change.

Test Plan

• uv run pytest — 86 passed, 100% coverage maintained
• uv run ruff check src/ tests/ — clean
• uv run mypy src/mt5_pnl_exporter — clean
• test_export_regenerates_when_prior_unreadable — garbage prior + succeeding account → exit 0, file regenerated and decryptable, warning emitted
• test_export_overwrites_unreadable_prior_when_all_fail — garbage prior + failing account → exit 1, readable all-error snapshot replaces the garbage
• Manual (Windows host): with a snapshot on disk, run set-encryption-passphrase to change the passphrase, then export — now warns and writes a fresh snapshot instead of crashing

Follow-ups (out of scope, noted from review)

• Tests exercise the decrypt-stage failure (garbage bytes); the catch covers later stages (corrupt-JSON, schema-mismatch) by type (ValidationError ⊆ ValueError). A test variant for those would lock the contract explicitly.
• Pre-existing: snapshot.read() does path.read_bytes() outside its try, so an OSError/PermissionError race (e.g. a locked file on Windows) still escapes uncaught — separate from this fix.

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