Per-subcommand --help and bench discoverability guardrails

[rymalia]

Background

Documentation for these surfaces is being handled in the CLI/MCP reference PR (#715),
but two issues need code changes, which is what this issue tracks:

1. --help is identical for every command. Every --help invocation
   (qmd query --help, qmd get --help, qmd collection --help, qmd embed --help,
   …) emits the same ~97-line block. There is no per-command help; the EBNF grammar
   appears even for embed and mcp where it's irrelevant.

2. qmd bench fails silently. If the fixture's collection isn't indexed, bench
   runs all queries to completion and prints a wall of 0.00 with no warning — ~2
   minutes of wasted work that reads as "search is broken." The usage message also
   points to a source path (src/bench/fixtures/example.json) that npm/npx users
   can't reach, and the fixture isn't shipped in the package.

Proposed changes

Progressive --help

Adopt the tiered/contextual help pattern used by uv, cargo, gh, rg:

• Top-level qmd --help = command directory (commands + one-line descriptions +
  global options).
• Per-command --help = only that command's flags, usage, and 1–2 examples.
• EBNF grammar lives in query --help only.
• Add --intent and --from to the relevant per-command help (currently missing
  from --help even though they work).
• Keep help text adjacent to each command handler so community PRs don't drift; add a
  test that asserts each help block's mentioned flags exist in the arg parser.

Note: bare qmd collection (no args) already shows a subcommand directory — extend
that pattern rather than replacing it.

bench guardrails

• Validate the fixture's collection exists before running; error early with the
  setup hint, mirroring qmd search -c <missing>.
• Warn on all-zero results (collection likely unindexed; suggest
  qmd ls <collection>).
• Make the example reachable for installed users — qmd bench --example to print
  the fixture JSON to stdout, and/or ship the example fixture as package data so the
  source-path reference resolves outside a git checkout.

Scope / non-goals

• Doc-only changes are handled separately in docs: CLI reference, collection-flag semantics, MCP params, bench, new commands #715.
• No changes to search behavior, scoring, or the bench metrics themselves.

References

• clig.dev help-output guidelines; uv/cargo/gh/rg precedent
• Companion docs PR: docs: CLI reference, collection-flag semantics, MCP params, bench, new commands #715