docs: document every undocumented config key + state file in .crosslink/

[dollspace-gay]

Summary

Closes GH#582 and the follow-up scope expansion. The original issue listed 19 undocumented keys in .crosslink/hook-config.json; this PR documents those plus every other undocumented key crosslink reads, plus every other user-editable file in .crosslink/.

Commit 1 — original 19 hook-config keys (73345d5c)

Appended to docs_src/reference/hook-config.qmd:

• 5 keys into ## Fields: tracker_remote, external-cache-ttl, external-url-ttl, repo-alias, sandbox.command
• New ## Agent overrides section with agent_overrides.{tracking_mode, blocked_git_commands, gated_git_commands, agent_lint_commands, agent_test_commands}
• New ## Sentinel configuration section with 13 keys grouped under top-level / sources / default_agent / escalation

Plus a "Local sandbox alternative" subsection in docs_src/guides/container-agents.qmd cross-linked to the new sandbox.command entry.

Commit 2 — additional 7 keys (940a1d14)

Found by greping every config.get(...) / config[...] site in src/ and .claude/hooks/:

• 5 keys in a new ## Watchdog configuration section: watchdog.{enabled, staleness_secs, max_nudges, check_interval_secs, grace_period_secs} (read by read_watchdog_config in kickoff/launch.rs)
• crosslink_binary and context_budget_chars appended to ## Fields (read by the Python hooks)

Commit 3 — new state-files reference page (6c3df2d4)

Per the user's expanded directive ("document everything undocumented, nothing is out of scope"), added docs_src/reference/state-files.qmd covering every remaining file in .crosslink/:

• hook-config.local.json — per-developer overlay
• agent.json — machine-local agent identity (all 7 fields)
• locks.json — coordination lock file including the previously-undocumented settings.stale_lock_timeout_minutes
• swarm.toml — trust model configuration ([trust] / [ignore] / [boundaries] tables)
• session.json — daemon-flushed session state mirror

Plus a closing table briefly acknowledging 14 internal state items (databases, logs, PIDs, caches, keys, subsystem dirs) so they're not invisible.

Registered the new page in both navbar and sidebar via _quarto.yml. Cross-link from hook-config.qmd to the overlay file. Explicit {#sentinel-enabled} and {#hook-config-local-json} anchors so cross-references through dotted-key headings stay stable.

Coverage

Documented keys in hook-config.qmd after this PR: 37, up from the 11 baseline (verified exhaustive by grep). New state-files page documents 5 user-editable files with full schemas plus an internal-state table covering the rest.

Test plan

• All key/default values cross-checked against the source-of-truth: config_registry.rs, resources/crosslink/hook-config.json, src/identity.rs, src/locks.rs, src/trust_model.rs, src/daemon.rs, WatchdogConfig::default() in kickoff/types.rs
• Cross-links between hook-config.qmd and state-files.qmd use explicit anchors for any dotted-key target
• _quarto.yml navbar + sidebar both register the new page
• Render the quarto site locally (or rely on the CI docs build) to verify the new page lands and all anchors resolve

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