docs: move specs and ADRs under docs/

[koistya]

Summary

Reorganizes documentation structure by moving specs/ into docs/specs/ and splitting the monolithic adrs.md into individual ADR files under docs/adr/.

Rationale

Why consolidate under docs/?

• VitePress serves content from docs/—having specs outside creates fragmented navigation
• Single source tree simplifies doc site configuration and cross-linking

Why split ADRs into individual files?

• The 700+ line adrs.md was hard to navigate, review, and extend
• Individual files (e.g., 003-explicit-ownership-verification.md) are easier to reference in code and PRs
• Follows the original ADR format intent (one decision per record)
• Enables linking directly to specific decisions

Changes

• specs/ → docs/specs/ (interface.md, backend specs)
• specs/adrs.md → individual files in docs/adr/ (ADR-003 through ADR-016)
• New docs/adr/000-template.md for future ADRs
• Simplified docs/specs/README.md with essential guidance
• Updated internal references (CLAUDE.md, backend READMEs)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.88%. Comparing base (97296e0) to head (6be2562).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   75.88%   75.88%           
=======================================
  Files          42       42           
  Lines        2658     2658           
=======================================
  Hits         2017     2017           
  Misses        641      641           

Flag	Coverage Δ
contracts-firestore	47.96% <ø> (ø)
contracts-postgres	43.81% <ø> (ø)
contracts-redis	40.96% <ø> (ø)
e2e	59.82% <ø> (ø)
unit	63.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
common/auto-lock.ts	75.77% <ø> (ø)
common/backend-semantics.ts	75.60% <ø> (ø)
common/backend.ts	100.00% <ø> (ø)
common/constants.ts	100.00% <ø> (ø)
common/crypto.ts	89.28% <ø> (ø)
common/disposable.ts	63.10% <ø> (ø)
common/telemetry.ts	67.83% <ø> (ø)
common/time-predicates.ts	100.00% <ø> (ø)
common/validation.ts	96.55% <ø> (ø)
firestore/backend.ts	100.00% <ø> (ø)
... and 15 more

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