…s 2h + 3) (#52)

Closes #42. Final two steps of the RFC migration.

Step 2h — holographic health_check:
  Holographic is local-only (SQLite); no remote service. But a
  read-only HERMES_HOME (RO mount, wrong perms) would still
  break it at runtime. Override verifies db_path's parent
  directory is writable.

    (True, "")                     — parent mkdir + W_OK pass.
    (False, "unreachable: <msg>")  — mkdir failed or W_OK denied.
    (False, "config_error: <msg>") — get_hermes_home raised.

Step 3 — doctor branch collapse:
  Every shipped provider now implements health_check() with the
  RFC #42 reason-prefix taxonomy. The provider-specific Honcho +
  Mem0 elif blocks (~120 lines combined) are gone. The unified
  dispatch is one ~80-line block in doctor.py keyed on the prefix:

    healthy → check_ok("<name> reachable")
    no_api_key / no_credentials → _fail_and_issue (setup hint)
    no_url / no_endpoint        → _fail_and_issue (URL hint)
    no_config                   → check_warn (run setup)
    disabled                    → check_info
    sdk_missing                 → _fail_and_issue (plugin docs)
    auth:                       → _fail_and_issue (rotate key)
    not_found:                  → _fail_and_issue (verify base URL)
    http:                       → check_warn (unexpected status)
    unreachable:                → check_warn (transient hint)
    config_error:               → check_warn (config raised)
    health_check_raised:        → check_warn (provider bug; RFC
                                   #42 says health_check MUST NOT
                                   raise, so this is a contract
                                   violation worth flagging)
    other                       → check_warn (unknown verbatim)

Doctor's Memory Provider section is now ~80 lines instead of
~200, and adding the 9th provider requires zero doctor changes.

Migration table (8 providers total):
  mem0       — PR #44  GET /v1/memory profile
  honcho     — PR #45  get_honcho_client handshake
  byterover  — PR #46  brv status (CLI + login)
  supermemory — PR #48  client.profile probe
  openviking — PR #49  /health endpoint probe
  retaindb   — PR #50  /v1/memory/profile GET
  hindsight  — PR #51  mode-dependent (local import / cloud /version)
  holographic — this PR  db_path parent writability

Tests:
  - 6 new tests for holographic health_check
  - Updated 3 doctor tests to assert the new unified dispatch
    (auth → _fail_and_issue; unreachable → check_warn; healthy
    → "<name> reachable") instead of the now-removed elif-block
    output strings.
  - 228 health_check + doctor tests pass total.

The #42 RFC is now fully implemented across all shipped memory
providers. Closing the issue with this PR.