spec 031-database-abstraction: 2958-line spec.md should be split, embedded agent-identity subsystem has no external references

[bug-ops]

Description

`specs/031-database-abstraction/spec.md` is 2958 lines. The specs quality audit in PR #6629 found it bundles multiple concerns that should be separate documents:

• Section 18, "Agent Identity in the Shared Data Model" (~900 lines, roughly lines 1941-2848), is a fully self-contained, independent subsystem (multi-tenant agent-scoped data isolation) with zero references to or from it anywhere else in `/specs/`.
• Sections 1-17 contain a superseded RFC-style narrative with the feature-flag design restated multiple times and version numbers that had drifted from `Cargo.toml` over time (partially corrected in PR docs(specs): correct stale references, broken links, and formatting #6629, but the narrative duplication itself was left as-is).
• Sections 19-20 (amendment log, implementation state) are accurate and should remain as the living reference.

A single 2958-line file is too large to serve as a coherent single implementation contract per the sdd skill's own size guidance, and section 18 in particular reads as a spec that was never given its own directory.

Expected Behavior

• Section 18 ("Agent Identity in the Shared Data Model") extracted into its own `specs/0NN-agent-identity-data-isolation/spec.md`, registered in `specs/README.md` and `specs/MOC-specs.md`, with a cross-reference left in `031-database-abstraction/spec.md`.
• Sections 1-17's restated RFC narrative consolidated to remove duplication, keeping one canonical statement of the feature-flag design.

Actual Behavior

Everything lives in one file, making it hard to review as a single contract and burying an independent subsystem's spec inside an unrelated document.

Environment

• Version: main branch, spec audit as of PR docs(specs): correct stale references, broken links, and formatting #6629
• Features: n/a (documentation only)

Logs / Evidence

See PR #6629 description, "Flagged, not executed" section, item 1.