docs(contributing): drop stale MCP-tool ToC entry, align just ci docs

[SeanGearin]

Problem

Two verified pieces of contributor-docs drift:

1. Dead ToC entry. The CONTRIBUTING.md Table of Contents lists
   "How to Add a New MCP Tool" (#how-to-add-a-new-mcp-tool), but no such
   section exists in the body — the body jumps from "How to Add a New Event
   Kind" straight to "How to Add a New API Endpoint". The section was
   deliberately removed in chore: deprecate sprout-mcp — fill CLI gaps, remove crate and all references #850 ("chore: deprecate sprout-mcp — fill CLI
   gaps, remove crate and all references"); the ToC entry was left behind
   and now links to a dead anchor.

2. just ci described inconsistently everywhere it is enumerated. The
   Justfile recipe (line 258) actually runs:
   ci: check test-unit desktop-test desktop-build desktop-tauri-check desktop-tauri-test web-build mobile-test
   (where check = fmt-check + clippy + desktop/Tauri/web/mobile lint
   checks). Four prose descriptions each claimed a different, incomplete
   subset:

   • CONTRIBUTING.md "CI Gate" comment: "check + unit tests + desktop build
     • Tauri check + mobile tests" (omits desktop/Tauri tests, web build)
   • CONTRIBUTING.md PR checklist: "fmt + clippy + unit tests + mobile"
     (omits desktop, Tauri, and web entirely)
   • TESTING.md troubleshooting table: "fmt, clippy, unit tests,
     desktop/web builds" (omits mobile and desktop/Tauri tests)
   • AGENTS.md Quality Gates: "fmt + clippy + desktop lint + unit tests +
     builds" (omits web and mobile)

   Since "PRs that fail just ci will not be merged", contributors deserve
   an accurate description of what the gate covers.

Fix

• Remove the stale ToC entry and renumber the two entries after it.
  Restoring the section was considered and rejected: the current
  crates/buzz-dev-mcp crate is a local dev-tools shim (rg / read_file /
  shell / str_replace), unrelated to the removed sprout-mcp agent API
  surface that section documented, and it has no README to source a
  replacement from. Removal matches the intent of chore: deprecate sprout-mcp — fill CLI gaps, remove crate and all references #850.
• Rewrite all four just ci descriptions to match the recipe: check
  (fmt + lints across Rust, desktop, Tauri, web, and mobile), unit tests
  (Rust, desktop, Tauri, mobile), and the desktop and web builds.

Test evidence

Docs-only change — no code paths touched, no build required per
CONTRIBUTING's own guidance.

• Verified no section heading nor anchor reference to "How to Add a New MCP
  Tool" remains anywhere in the repo's markdown after the change
  (grep -rn 'how-to-add-a-new-mcp-tool\|How to Add a New MCP Tool' --include='*.md' . → no matches).
• Verified every remaining just ci enumeration against the Justfile
  recipe at line 258 and its sub-recipes (check line 94, test-unit,
  desktop-test, desktop-build, desktop-tauri-check,
  desktop-tauri-test, web-build, mobile-test).
• All other ToC entries were checked against body headings; no other dead
  anchors exist.

Links

• chore: deprecate sprout-mcp — fill CLI gaps, remove crate and all references #850 — the commit (f1c672f) that removed the sprout-mcp crate and the
  "How to Add a New MCP Tool" section but missed the ToC entry.
• Related but non-overlapping: open PR docs: prepare project documentation for public release #2228 repoints the CONTRIBUTING.md
  "GitHub Discussion" pointer to Issues (the repo has Discussions
  disabled). This PR deliberately does not touch that line to avoid
  duplicating docs: prepare project documentation for public release #2228.