feat(explore): markdown index (#361) with a section-first doc tier

[bompus]

Follow-up to #361 and #1439, with the measurement from this comment on #361.

What this carries. #361's markdown extractor as it stands (headings, sections, tables and links as nodes; name-matcher and resolution hooks; its five test files), rebased onto main, plus a doc tier in codegraph_explore and three fixes the index needs to leave code answers alone.

The doc tier. A doc-shaped query that names a markdown file renders that file's best sections first and whole: the top three by idf-weighted line hits, where a term that also appears in the file's own path weighs zero (it located the file, not the line) and a heading the query covers word for word counts as a named section; 8k characters per file, spent in score order and rendered in file order. The blast-radius, relationships and "additional files" blocks stay off unless a code file rendered too. Code queries take the same path they did before.

Three fixes for a shared index.

• detectGeneratedFile skips the header check for markdown: a README that quotes "generated by" was dropped from the index.
• Markdown nodes leave the subgraph unless the doc tier seeded them: section bodies share the FTS table with code, so an English word in a README matched into code queries.
• The explore budget tiers count code files only. Markdown took a 466-file repo to 575 indexed files, across the 500-file breakpoint, and every code answer grew a Relationships block.

The server instructions now say markdown is indexed; #361's text still listed docs under what codegraph does not index, and measured as merged the model never called explore for a doc question (24 cells, 0 calls).

Measured. Headless Claude Code, Opus, a repo with 109 markdown files, six doc-question tasks, three rounds of 12 fresh cells against the shipped build with the same repo rules:

	Round 1	Round 2	Round 3
Correct	12 / 12	12 / 12	12 / 12
Tool calls, median (shipped: 4)	1	1	1
Explore adopted	10 / 12	11 / 12	10 / 12
No Grep or Read after explore	8 / 12	9 / 12	9 / 12

Every explore call chose the right file and section; the misses are the model grepping a file the prompt already names, and one prompt it reads as being about its own scheduling tools. Cost per cell $0.30 against $0.54.

Code answers on the same repo, one fresh session per query, 25 code queries against main: 11 byte-identical, 3 the same lines reordered, 10 swap a fourth- or fifth-ranked padding file (markdown documents shift FTS tie ranks), and one prose prompt with no identifier answers from two markdown files instead of five unrelated code files.

Tests. tsc clean. #361's five test files plus the two explore tests that the raw patch broke on main (explore-allocation-1500, explore-session-state): 797 pass; the failures on my Windows host are all EBUSY temp-dir teardown in afterAll, none an assertion.

Re-index after upgrading: the markdown nodes are written while indexing.