CC-490: scope explicit memory configuration by project

[screenleon]

Summary

• add project-scoped explicit memory directory selection keyed by repository
• fail closed across resolve, append, context, doctor, directory lookup, shard, rebuild, and routing-log migration when a matched explicit selection is invalid
• expand cross-project isolation and consumer-level regression coverage

Why

A single global explicit memory directory could silently route multiple repositories into the same canonical store. Some compatibility and maintenance paths could also fall back to populated legacy Claude memory after a matched project configuration became unavailable.

This change makes repository identity part of explicit memory selection and prevents those fallback paths from mutating or reporting the wrong store.

Validation

• Claude PR gate with explicit --test-cmd: GO (gate-20260715-032942-90cd0b)
• authoritative full suite after gate: 79 passed, 0 failed, 0 skipped
• state-bound full-suite artifact verified for the implementation tree
• subsequent backlog-only closure commit was linted and intentionally did not rerun gate/full suite per maintainer direction

Follow-up

CC-491 remains separate and should be handled next to prevent QA reviewers from rerunning preflight tests without evidence-based need.