docs: fix stale line counts, version, endpoint name, and architecture narrative (#872)

[vivekchand]

Summary

Fixes all items flagged in the weekly docs-drift sweep (#872). All three doc files still described the old single-file monolith; this PR brings them in sync with the current per-feature Blueprint layout and actual file sizes.

Changes

CLAUDE.md

• Update five stale line-count figures: dashboard.py 25,400→15,013; routes/sessions.py 1,190→1,556; routes/brain.py 800→1,027; clawmetry/sync.py 3,000→3,876; clawmetry/config.py 58→79
• Replace /api/brain endpoint entry with the two actual routes: /api/brain-history + /api/brain-stream
• Bump __version__ reference from 0.12.99 → 0.12.162

ARCHITECTURE.md

• Rewrite the "Single-File Architecture" section to describe the per-feature Blueprint layout (the single-file claim has been architecturally false since routes/ was introduced)
• Fix history.py snapshot cadence: "5 minutes" → "60 seconds" (matches POLL_INTERVAL = 60 in the code)

CONTRIBUTING.md

• Update "Make Changes & Test" step to mention routes/<feature>.py alongside dashboard.py
• Expand project-structure tree to include routes/ and clawmetry/ directories
• Replace the "Breaking the single-file architecture" avoid-bullet with the accurate equivalent
• Update Philosophy paragraph to match the current layout

Test plan

• make lint — no Python files changed, syntax check passes unchanged
• Read each updated section and confirm numbers match wc -l output on the actual files
• Confirm routes/brain.py has no /api/brain route (only /api/brain-history and /api/brain-stream)
• Confirm dashboard.py __version__ is 0.12.162

Bot meta

Draft PR opened autonomously based on the plan in #872. Marked draft for human review — mark Ready for Review once happy.

Closes #872

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