Harden install and upgrade docs

[jeffhuber]

Part of #536. Fixes #542.

Summary

• add a cold-install vs upgrade section to the install guide
• document exact pipx upgrades, uv reinstalls, pipx-to-uv migration, and sandboxed pipx directories
• put the install matrix directly in the 10-minute guide
• align quickstart Codex smoke/API-key auth guidance with runner docs
• add troubleshooting for stale command paths and loopback-only Board URLs
• add release-hygiene coverage for the key install/upgrade snippets

Verification

• scripts/dev-python -m unittest tests.test_release_hygiene.ReleaseHygieneTests.test_install_and_upgrade_docs_cover_agent_paths tests.test_release_hygiene.ReleaseHygieneTests.test_codex_smoke_docs_use_supported_flags
• scripts/dev-python -m pytest tests/test_release_hygiene.py
• scripts/dev-python -m ruff check .
• scripts/dev-python -m pytest
• git diff --check
• scripts/dev-python scripts/privacy_scan.py

Privacy

Docs/tests only. No source, diffs, transcripts, raw stdout/stderr, auth output, local paths, or secrets are uploaded or added by this change.

[gitar-bot]

Code Review ✅ Approved

Hardens install and upgrade documentation with cold-install vs upgrade guidance, exact package manager procedures (pipx upgrades, uv reinstalls, migration paths), install matrix in the quickstart, aligned authentication examples, troubleshooting for stale paths and loopback Board URLs, and release-hygiene test coverage for install/upgrade snippets. No issues found.

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[jeffhuber]

Claude audit (merge-authority lane)

Head SHA: c3102e0d7fb89a42c9b29528c8985eb3cdada765
Findings: P0=0, P1=0, P2=0, P3=0 (blocker policy: any P0/P1/P2 -> BLOCKED)

Claude Audit: PASS

Summary:

Docs-only PR (README, install/quickstart/troubleshooting/try-in-10-minutes) plus a new self-verifying test that asserts specific strings exist in those docs. Checked every assertion in test_install_and_upgrade_docs_cover_agent_paths against the actual diff content; all match. No code paths changed, no security or data-loss risk introduced. No trusted project context to contradict.

Findings: none.