refactor(cli-schema): derive the flag tail of usage synopses so an option addition never edits usageOverride

[thymikee]

Wave 2 child of #2409. Named but not done in PR #2421.

Purpose

An option added to a command whose synopsis is hand-written must be restated in that synopsis by hand. #2421 made the descriptor the single declaration of an option for cli-schema flags, the MCP schema and daemon flags; the usageOverride string is the last restatement left on the help surface.

Baseline

On main at 41e2633f10:

• buildCommandUsage returns schema.usageOverride verbatim when it is set (src/cli-schema/usage.ts:19). Otherwise it renders the positionals plus one [label] per key in allowedFlags, in that order, using the flag declaration's usageLabel.
• 39 command descriptors set usageOverride (grep -rn "usageOverride:" src/commands). 38 encode shapes the generator cannot express: alternation (clipboard read | clipboard write <text>), subcommand forms (session list | session state-dir | …), value enumerations (orientation <portrait|…>), multi-line families (perf, metro).
• snapshot is the one override whose content is flags-only (src/commands/capture/snapshot.ts:54). It differs from the generated line in two ways. The generator renders --depth, -d <depth> and --scope, -s <scope> (the declared usageLabel, src/commands/cli-grammar/flag-definitions-workflow.ts:185) where the override shows -d <depth> and -s <scope>. The generator appends [--record] because record is in allowedFlags (snapshot.ts:56), while no synopsis in website/docs/docs/commands.md shows [--record] today (0 occurrences).
• Replay of feat(ios): expose AX custom actions on merged accessibility elements #1665 (--actions on snapshot): the change edited the override string and the docs line website/docs/docs/commands.md:353, which src/__tests__/command-doc-coverage.test.ts:78 pins byte-for-byte against buildCommandUsage('snapshot', …).
• Help tests are literal assert.match asserts, not golden files (src/cli-schema/cli-help-command-usage.test.ts:270 and siblings). None pins the full snapshot line.

Required work

1. Decide the two rendering rules on the flag declaration, not per command: (a) how a flag with a short alias renders in a synopsis (full --depth, -d <depth> as generated lines do today, or short-only as the snapshot override does), and (b) which cross-cutting flags stay out of synopses. Encode (b) as a field on the flag declaration (for example usageHidden: true on the record definition), never as an override.
2. Drop usageOverride from snapshot. Its --help line is generated; the docs line at commands.md:353 moves to the generated form and the byte-for-byte doc test stays green.
3. Classify the remaining 38 overrides in the PR body: "positional grammar the generator cannot express, keep" or "flag tail derivable". Where an override is a hand-written positional part followed by flags, split it so only the positional part stays hand-written and the flag tail is generated from allowedFlags. Pure alternation and subcommand overrides stay as they are.
4. Guard: a test fails when a command has usageOverride set and a key in its allowedFlags that is neither marked hidden nor mentioned in the override. An override can no longer silently lag its flags.

Acceptance, measurable from git

• Re-doing feat(ios): expose AX custom actions on merged accessibility elements #1665's --actions addition on the new shape edits no usageOverride string. The docs line still changes; that is the documented coverage rule, not a restatement.
• agent-device snapshot --help output is either byte-identical to today or changed exactly per rules 1(a) and 1(b), with the help tests updated in the same PR and the rule pinned by one test.
• Every remaining usageOverride mentions every non-hidden flag in its command's allowedFlags (the step-4 guard is green), or the command is listed in the PR as an intentional exception with the reason.

Non-goals

• A usage grammar for alternation or subcommands; the non-derivable overrides stay hand-written.
• Changing option names or semantics. listUsageOverride is untouched.

Dependencies

None. Builds on #2421 (ad79461c72).