fix(cursor): centralize platform-aware Cursor storage path resolution

[beruro]

Problem

Cursor storage paths were resolved independently in agent_cli and orgtrack_core. One implementation was macOS-only and fabricated /Users/_unknown when home discovery failed; the other had a broader but separate platform matrix. The duplicate ownership produced incorrect Linux/Windows behavior and inconsistent override handling.

Solution

• Add the canonical resolver under the workspace's existing leaf crate, app_paths::cursor, and make both consumers delegate to it.
• Resolve macOS, Linux/XDG, and Windows/APPDATA layouts with explicit fallbacks.
• Preserve ORGII_EXTERNAL_HISTORY_HOME isolation and keep the Cursor plugin cache home-anchored.
• Return typed unavailability instead of manufacturing a filesystem path; both consumers degrade to their existing "Cursor not installed" empty state.
• Implement the core as a pure CursorEnv plus platform resolver so every platform and override path can be tested on any host.

The branch is merged with the current develop; GitHub reports it mergeable.

Potential risks

• Linux now honors absolute XDG_CONFIG_HOME, and Windows honors relocated %APPDATA%; this intentionally changes edge configurations to match Cursor/Electron behavior.
• agent_cli is now isolated by ORGII_EXTERNAL_HISTORY_HOME, consistent with other external-history discovery but different from its previous real-home lookup.
• Missing-home behavior now returns None immediately instead of probing a fabricated path whose existence check failed; the user-visible result is equivalent.
• Other Cursor-path duplicates remain outside this PR and are follow-up sweep candidates.
• There is no persistence, schema, wire, or dependency migration. Rollback is a normal revert to the two prior resolvers.

Audit

Architecture review covered path ownership, dependency direction, platform/config precedence, typed failure handling, caller compatibility, and initialization parity. The canonical leaf-crate placement avoids a new agent_cli -> orgtrack_core dependency edge.

Verification

• cargo test -p app_paths — PASS, 21 passed / 0 failed, including the 13 Cursor-path matrix tests.
• cargo check -p agent_cli -p orgtrack_core -p app_paths — PASS.
• git diff --check — PASS.
• Merge of current origin/develop — PASS, no conflicts; checks were run after integration.