Document the /aviator help slash command

[brain-crystal]

/aviator help was missing from the docs. This documents it across the pages where a reader would look for it.

• mergequeue/reference/slash-commands.md — new ## Help section as the first section: what the command posts, an info hint that the list is not filtered by enabled products (and that a command for a disabled product will not perform the action), and the unrecognized-command suggestion. Frontmatter description: extended to mention help.
• mergequeue/how-to-guides/slash-commands.md — one paragraph after the /aviator refresh example pointing at /aviator help, linking to the reference page.
• flexreview/reference/flexreview-slash-commands.md — top info hint pointing at /aviator help, plus ### Refresh and ### Breakglass sections in the existing Suggest/Assign style. Breakglass links to the existing breakglass scenarios anchor.
• runbooks/how-to-guides/providing-feedback.md — one line noting /aviator revise also appears in the help output.

Verified against aviator-co/mergeit, src/core/slash_commands.py: the help groups (MergeQueue, FlexReview, Runbooks, Git Operations), that _handle_flexreview_refresh dispatches both process_fr_teams and process_fr_approvals, and that --reason on breakglass is required=True.

Two things worth knowing, neither addressed here:

• The help text is a static string and has drifted from the dispatch table. The table registers validate-config, stack merge, stack cancel and merge --targets, none of which help lists. It drifts the other way too: help advertises merge --skip-line / --skip-validation and backport --use-new-backport, which the reference page does not document. That is a backend fix, not a docs one.
• The reference page describes the unrecognized-command reply as Unknown /aviator command rather than quoting it verbatim, since the real string interpolates the typed command.

https://claude.ai/code/session_011oFBnjaoTAAv8MZhoy5Mdm