docs(web): add boy-scout decomposition convention for accreted modules

[staging-tembo]

Summary

Encodes the methodology from #311 into web/AGENTS.md so coding agents follow it by default when working in web/.

New section — "Big grown-by-accretion modules: carve out, don't pile on":

• Boy-scout, not big-bang: no dedicated refactor PRs; when a feature touches one of the accreted multi-domain modules, extract the domain being touched into its own module as part of that change instead of adding to the pile.
• Concrete split patterns per module shape:
  • settings/actions.ts → per-domain action files (re-exporting from the original if needed for import stability)
  • runs-db.ts / workspace.ts → split along query-subject lines
  • large client components (agents-inventory.tsx) → extract the self-contained sub-tree being modified
• Mechanical move + import update in the same PR as the motivating feature, with no behavior changes bundled into the move, so review cost stays near zero and the feature's own testing covers it.
• Guardrails so the rule doesn't over-trigger:
  • data-heavy files (e.g. mcp-providers.ts, ~95% flat catalog data) are exempt — length is not the risk signal, unrelated logic domains are
  • ~1,500 lines of logic is the escalation point back to Boy-scout decomposition of grown-by-accretion web modules #311 for a scheduled split

The section links to #311 for the live offender table rather than duplicating it — line counts drift, and the issue stays the tracking source of truth.

Docs-only change: web/CLAUDE.md already imports AGENTS.md, and the root AGENTS.md already directs agents to read the subdir file, so no other files need touching.

Refs #311

Want tembo to make any changes? Add a comment with @staging-tembo and i'll get back to work!

[staging-tembo]

Requesting review from @ryw who has experience with the following files modified in this PR:

• web/AGENTS.md