Group tab commands under tabs, scope flags per command

[colevels]

Summary

run and serve shipped as top-level commands in #22 but never made it into a released binary — v0.5.3 is tagged at the commit before it. No user has them, so they're removed outright: no aliases, no deprecation warnings.

ACCOUNT  login · logout · whoami
DOCS     docs push <file> · docs list · docs open <id>
TABS     tabs check <file> · tabs push <file> · tabs serve · tabs list · tabs prompt
SETUP    init · update · help

• run → tabs push. The old name claimed something the command cannot do: it has no browser access and only queues a script for you to click Run on — which the README had to disclaim twice. Someone typing tabbrew run close-duplicates.tbrew and seeing ✓ Script queued could reasonably believe their tabs had already closed. docs push ↔ tabs push now read as one mental model.
• serve → tabs serve, so the whole tab workflow lives under one noun.
• New tabs list — the counterpart to docs list. tabs serve wrote a file and said nothing more about it; the only way to see the result was cat | jq. Leads with the export's age, since the ids go stale.
• New src/registry.ts — the command surface as data. printHelp renders from it and assertFlagsAllowed validates against it, so help can't drift from the routing and one command's flag can't leak into another.

Fixes a real bug

parseArgs takes one flat option table, so --port was accepted by every command and read by only one. With a serve on a non-default port:

$ tabbrew run script.tbrew --port 49299
✓ Script queued (2 op(s))          # …delivered to 49227 instead

tabs push now takes --port, and both commands resolve it through a single resolveServePort() — they can't disagree. Off-command flags are now rejected:

$ tabbrew docs push --port 99
✗ --port is not an option for `docs push`.
  `docs push` accepts: --cloud, --title

Also

• init --skill <variant> → --variant — same three prompts as tabs prompt --variant; --no-skill stays the install-or-not switch. Bad variant now throws a typed error instead of a bare Error.
• tabs push rejects a zero-op script locally instead of surfacing the server's invalid_payload, which read as a bridge failure. (Slightly beyond the agreed scope — but the confusing message came from the error path this PR rewrites, so shipping it as-is meant knowingly keeping a bad message.)
• Help is grouped, the header line no longer overflows 80 columns, and ENVIRONMENT is split into "things you might set" vs. endpoint overrides.
• tabs serve startup prints what to do next in Chrome instead of a raw route table (routes still shown under TABBREW_DEBUG).
• Table helpers → src/table.ts, shared by docs list/tabs list. Unchanged logic: still measures display width via Bun.stringWidth.

Compatibility

The wire protocol (POST /tabs, GET|POST /script, GET /health) and port 49227 are untouched, so the extension keeps working against this build. Its UI copy still names the old commands — filed as colevels/tabbrew#92, to fix before either side's next release.

Test plan

• bun run typecheck and bun run build pass
• run/serve are gone (Unknown command); --skill rejected
• tabs serve --port 49299 + tabs push --port 49299 round-trips — verified the script lands on 49299, which is the case that previously misdelivered
• tabs push to a port with no listener, and with an empty script, both give actionable errors
• Flag scoping rejects docs push --port / tabs list --port
• tabs list against Thai + Japanese + untitled tabs — columns verified equal by measured display width, not by eye
• init / init --variant compact / --dry-run / --uninstall run from the compiled binary (exercises the SKILL.*.md text imports); installed TABBREW-CLI.md contains no dead command names

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