Cowork internal refactors — DRY and hot-path cleanup

[sergio-sisternes-epam]

Follow-up to #913. Surfaced by the APM Expert Review Panel (Python Architect specialist). All MEDIUM-severity maintenance items, no user-facing impact. CEO arbitration: acceptable as follow-ups, no release gate.

1. SkillIntegrator inlines dynamic-root routing 3x

File: src/apm_cli/integration/skill_integrator.py:666-667, 778-779, 838-839

Three call sites open-code if target.resolved_deploy_root is not None: ... instead of calling the target.deploy_path() accessor the PR itself introduced. This re-duplicates the very abstraction the target profile is meant to provide.

Fix: route all three sites through target.deploy_path(...).

2. resolve_cowork_skills_dir() called twice per entry in sync_remove_files

File: src/apm_cli/integration/base_integrator.py around lines 410 and 420

Each call hits apm config read + OneDrive glob. In a sync loop this compounds.

Fix: resolve once at the top of the cowork branch in sync_remove_files, reuse the value.

3. unset_temp_dir() / unset_cowork_skills_dir() copy-paste bypassing update_config()

File: src/apm_cli/config.py:129-140, 178-189

Two near-identical unset helpers, both re-implementing the read-modify-write pattern instead of calling update_config(). Maintenance trap for any future config-file format change.

Fix: consolidate into _unset_key(key) that calls update_config(); keep the two public wrappers as one-liners.

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Acceptance criteria

• All three SkillIntegrator sites call target.deploy_path(...).
• sync_remove_files resolves the cowork root once per sync invocation.
• Both unset helpers delegate to a single implementation that uses update_config().
• Existing tests remain green; add a regression test for the update_config code path in both unset helpers.

/cc panel review: #913