docs: refresh site and README for post-v0.1.0 accuracy

[timimsms]

Lands the verified accuracy fixes from the post-v0.1.0 documentation audits (live site + sources). Every claim was re-verified against the working tree before editing.

Fixes

1. Homebrew section: stale future tense (README.md:34, docs/site/index.md:32)
   Audit evidence: live homepage and README still said "Available with the first tagged release." while gh release view v0.1.0 shows the release published 2026-07-15 with 5 platform archives + checksums.txt and the tap live.
   Fix: "Installs the latest tagged release from the timimsms/homebrew-cu tap."

2. npm section: placeholder status not conveyed (README.md:42, docs/site/index.md:42)
   Audit evidence: @timimsms/cu exists on npm as a 0.0.1 placeholder (name reserved); "Planned as @timimsms/cu (not yet published)" could mislead readers who find it on npm.
   Fix: states the name is reserved as a placeholder only, functional package not yet published; still no install command advertised.

3. go install "dev" version caveat (README.md, docs/site/index.md, under the Go install block)
   Audit evidence: Makefile injects version via -ldflags only in make/goreleaser builds; a fresh go build from main reports cu version dev (open issue version: fall back to debug.ReadBuildInfo() so 'go install' builds report their real version #36).
   Fix: one-line note linking version: fall back to debug.ReadBuildInfo() so 'go install' builds report their real version #36.

4. Auth page: env-fallback cross-reference (docs/site/authentication.md:39)
   Audit evidence: "no environment-variable fallback" is accurate today (only os.Getenv in the tree is HOME) but reads as permanent; issue auth: support CU_API_TOKEN env fallback for headless/CI authentication #26 (open) tracks the planned fallback.
   Fix: appended "An environment-variable token fallback is planned — see issue auth: support CU_API_TOKEN env fallback for headless/CI authentication #26."

5. Edit pencil never renders (mkdocs.yml)
   Audit evidence: edit_uri: edit/main/docs/site/ is set but theme.features lacked content.action.edit, so live HTML contains zero edit links (grep count 0). Verified locally: after adding the feature, the built index.html contains the edit link (grep count 1).
   Fix: added content.action.edit to theme.features.

6. Deploy trigger broader than the published site (.github/workflows/deploy-docs.yml)
   Audit evidence: paths included docs/** while docs_dir is docs/site; run 29550158962 was triggered by PR docs: add context-layer design spec (refs, aliases, packs, onboard, pickers) #39 whose only file was docs/design/context-layer.md (intentionally unpublished).
   Fix: narrowed to docs/site/** (mkdocs.yml and the workflow file remain triggers).

7. mkdocs-material unpinned (.github/workflows/deploy-docs.yml:36, .github/workflows/ci.yml:133)
   Audit evidence: pip install mkdocs-material with no constraint + --strict builds; Material's MkDocs 2.0 advisory banner warns of upcoming breaking changes.
   Fix: pinned to 'mkdocs-material==9.*' in both workflows.

Deliberately not included

• Command-page regeneration: rebuilt cu from main and ran cu docs markdown; all 45 regenerated pages are byte-identical to docs/site/commands/ except the cobra footer-date line (0 non-footer diff lines). The live audit's "content drift" claim did not reproduce; regenerating would be 45 files of footer-date churn for zero content value.
• Changelog/release page, README release badge, LICENSE copyright line — feature proposals / maintainer-preference items, left for the maintainer.

Verification

• mkdocs build --strict in a scratch venv (mkdocs 1.6.1, mkdocs-material 9.x): exit 0, no warnings.
• Built HTML spot-checks: edit link present, new npm phrasing present.

🤖 Generated with Claude Code