[ponytail] _core shrink: SessionStore on LRUCache, parseDateHint grammar, schema/scan/config one-liners #120

Cut

Same logic, fewer lines, in src/_core.

Findings

• yagni: SessionStore re-implements TTL plus LRU eviction that src/_core/cache/LRUCache already provides (src/_core/search/session.ts:76). Back it with new LRUCache<string, RecallSession>({maxSize: 10, ttlMs: 30 * 60 * 1000}) and delete evictExpired, evictLRU and the manual lastAccessedAt bookkeeping (~45 lines).
• shrink: parseDateHint has 24 regex branches (src/_core/search/dates.ts:160). Keep every phrasing the recall tool description promises (today, yesterday, this/last week|month|year, N days|weeks|months ago, this day last year, in , , since , before , on this day, ISO date/month) and drop the rest ("this date in history", "every year on this day", "day before yesterday", spelled-out numbers, "10 march 2026") together with the word-number table and their tests (~120 lines).
• stdlib: pad() and toIsoDay() hand-format a UTC date (src/_core/search/dates.ts:57). toIsoDay(d) is d.toISOString().slice(0, 10); the remaining pad calls are String(n).padStart(2, "0") (~8 lines).
• shrink: CREATE TABLE IF NOT EXISTS schema_migrations is copy-pasted into all five checkpointed migrate functions (src/_core/db/schema.ts:608). Hoist into one ensureMigrationsTable(db) (~25 lines).
• shrink: migrateExpandedTypes keeps a SAVEPOINT + INSERT + ROLLBACK probe under the sqlite_master DDL fast-path that already answers the same question (src/_core/db/schema.ts:487, ~16 lines). Check issue [db] migrateExpandedTypes fails with FOREIGN KEY constraint failed on any database with relationships rows, leaves entities_new behind, and then fails every subsequent init #101 first; if the fix for it lands on the probe path, coordinate rather than delete blindly.
• shrink: the dedup branch in src/_core/search/scan.ts:245 rebuilds the numbered-context window inline, duplicating extractContext. Give extractContext explicit start/end arguments (~20 lines).
• yagni: describeModelCacheDir reverse-engineers the precedence resolveModelCacheLocation just computed (src/_core/config/index.ts:74). Keep the ModelCacheResolution on the config (~13 lines).
• yagni: resolveModelCacheDir is an exported one-liner with one caller (src/_core/embeddings/model-cache.ts:20). Inline (~6 lines).
• shrink: budgetResults is a deprecated alias for allocateBudget with two callers (src/_core/search/orchestrator.ts:39). Inline and drop the export (~12 lines).

Verification

npm run build and npm run test:run green before and after. The dates tests for every kept phrasing still pass; removed phrasings get their tests removed in the same commit and listed in the PR.