docs(cli): generated agent-discoverable command & flag reference (DRG-483)

[luke-speechify]

Adds full, agent-discoverable documentation covering every command and flag — generated from the CLI's own Commander definitions, so it can never drift from what the binary actually accepts.

What ships

• llms.txt — the llms.txt-standard index (title, summary, links to each command).
• llms-full.txt — every command and flag inline, in one file.
• docs/ — one page per command (login, logout, whoami, say, voices, voices list/get, api, mcp, mcp install) plus docs/global-options.md.

Each page has usage, arguments, command-specific options (with defaults, choices, required markers), and subcommand links.

How it's built

• src/docs/reference.ts walks the Commander tree and renders the docs deterministically (no dates, no version) so a checked-in copy can be diffed.
• pnpm docs:generate writes the files. A vitest drift guard fails pnpm test when the checked-in copies are stale — and CI runs typecheck/build/test (not lint), so the guard is actually enforced.
• Extracted a side-effect-free buildProgram() into src/program.ts; both bin.ts and the generator build the same tree.

Small CLI improvement

• Gave the positional arguments descriptions (say [text], api <endpoint>, voices get [voice-id]) so they now show up in both --help and the generated docs.

Discovery

Context7 already indexes the repo (context7.json), and docs/ + the llms*.txt files are not excluded — so these land in the hosted index for answer engines.

Testing

• 216 tests pass; drift guard verified (tampering a doc fails the suite).
• typecheck, biome, and tsup build all clean.