feat(#531): max_tokens two-step packing for codedb_context

[justrach]

Pick 5 of the #531 roadmap (token-budget-aware context packing) — step 2 of the two-step finish.

What

codedb_context gains max_tokens (~4 chars/token estimate, min 256). The handler now works in two steps:

1. Render: every section — reader.md, head, symbol definitions (a rich variant with inlined bodies plus a lean def-lines-only fallback), callers, calls, file list, snippets — renders into its own arena buffer.
2. Pack: sections are admitted by value order (head always → files → symbols rich-falling-back-to-lean → reader.md → callers → calls → snippets) until the byte budget runs out, then emitted in document order. Every omitted section leaves a one-line [max_tokens: omitted … (~N tokens)] marker so the agent knows what to raise the budget for or fetch directly.

So "give me the best ~2k tokens for this task" keeps the cheap high-value head (file ranking, symbol definition locations) and sheds the fat tail (snippet blocks, inlined bodies) — instead of truncating mid-section.

Compatibility

Without max_tokens every section is admitted and emitted in the original order — output is unchanged, and the pre-existing context tests (issue-570 fallback, etc.) pass untouched. The pure-render refactor also tightened the shared-lock scope to just the snippet-extraction block.

Tests

• issue-531: codedb_context max_tokens packs sections by value under the budget — fat symbol body + five mention files; asserts: unbudgeted output unchanged (no markers) and larger; budgeted output ≤ budget×4 + marker slack; head and ## Most-relevant files survive while ## Top sites is dropped.

Suite: 814/814.

🤖 Generated with Claude Code

[github-actions]

Benchmark Regression Report

Thresholds: 10.00% and 50,000 ns absolute delta

NOISE means the percentage threshold was exceeded, but the absolute delta was too small to fail CI.

Tool	Base (ns)	Head (ns)	Delta	Abs Delta (ns)	Status
codedb_bundle	115940	114577	-1.18%	-1363	OK
codedb_changes	11409	11126	-2.48%	-283	OK
codedb_context	2008097	2008144	+0.00%	+47	OK
codedb_deps	325	362	+11.38%	+37	NOISE
codedb_edit	42238	38080	-9.84%	-4158	OK
codedb_find	11231	11028	-1.81%	-203	OK
codedb_hot	26658	31763	+19.15%	+5105	NOISE
codedb_outline	38289	46889	+22.46%	+8600	NOISE
codedb_read	17913	17709	-1.14%	-204	OK
codedb_search	30135	29282	-2.83%	-853	OK
codedb_snapshot	78621	76220	-3.05%	-2401	OK
codedb_status	9805	9624	-1.85%	-181	OK
codedb_symbol	50724	53942	+6.34%	+3218	OK
codedb_tree	47230	46970	-0.55%	-260	OK
codedb_word	13332	12606	-5.45%	-726	OK