docs: list amp as a built-in target in the architecture and diagnostics references - #758

Closes #755. Closes #756.

Amp shipped as a built-in adapter in #729, but two documentation contracts still described the
four-adapter registry that preceded it, and both feed public pages.

website/docs/{en,zh}/guide/concepts/architecture.mdx (hand-written)

• amp joins the planner table, naming adapters/amp.ts and its pinned
  adapters/capabilities/amp-0.0.0-20260907001852-gf348fed.json.
• The createDefaultRegistry prose now includes amp and says what registration order is not:
  planComposite sorts the selection before planning, and the composite identity is the sorted
  name join. portable stays the default; plugin stays a non-target.
• One new paragraph on what makes Amp architecturally different — a generated index.js
  PluginAPI factory under .amp/plugins/{plugin}/ that registers Skills and callbacks through
  the API, and skill-scoped MCP (exactly one Amp Skill for a canonical server; a compiler-owned
  local server refused for want of a documented plugin-root placeholder). The per-surface
  evidence stays on the generated Hosts page, which the paragraph links, along with the
  author-facing capability map from docs: complete convention-first guides, targets, and capability navigation (EN/ZH) #754.
• "Four planners, one merge" becomes five, and BundleIdentityHost is corrected to
  amp | claude | codex | cursor with the note that DoctorHost excludes Amp.

docs/diagnostics.md (the source the site's diagnostics reference is rendered from)

• AB4100: the built-in registry statement and the recovery list both name amp.
• The pre-Amp enumerations found by re-reading the file: the skill-host lowering preamble,
  AB3006 (Amp's builtin-tools / mcpServers extension fields and targets.amp), AB3008
  (Amp documents no Skill Markdown interpolation engine, so it belongs with Codex, Cursor, and
  portable), AB3009 (foreign-placeholder detection covers Amp documents too), AB3010 (Amp
  validates the Agent Skills frontmatter schema plus its own skill MCP rules), the composite-root
  section, AB4106, and AB6023. The stale "unified plugin target's shared document" clauses
  went with them.
• docs/diagnostics.md line 239 keeps its four host-scoped config keys deliberately: the Amp
  adapter declares no configExtension, so claude, codex, cursor, portable is the
  complete list there, matching reference/configuration.mdx.

No generated page was hand-edited — website/docs/{en,zh}/reference/diagnostics.md is rendered
from docs/diagnostics.md at build time, and the built output was checked to confirm the
AB4100 row now lists all five targets in both locales.

New guard (packages/agent-bundle/tests/diagnostics-doc-targets.test.ts): the AB4100 row's
trigger and recovery target lists are parsed out of docs/diagnostics.md and compared with
createDefaultRegistry().names(), so the next built-in adapter fails a test instead of silently
drifting the public reference. Falsified before landing: removing amp from the row fails the
test with the target diff.

Docs and a test only — no publishable package source changed, so pnpm changeset status --since=origin/main requires nothing (skip-changeset).

Local gate

Run on f6b92e55a3, which contains current origin/main (361d27ff5e):

Deslop

Deslop: claude-fable-5-1-thinking-xhigh, 2 edits — trimmed an eight-line doc comment in the
guard test that narrated issue history, and removed the stale "unified plugin target's shared
document" clauses the Amp rows inherited.

Self-review

Reviewer: claude-fable-5-1-thinking-xhigh, then gpt-5.6-sol-max on the fix commit.