feat(index): the documentation pass follows the index mode by default

[DeusData]

Doc-section -> code candidates (doc_link_candidates, #2574/#2610) are the
documentation-in-the-graph feature of this release. They now run by
default for full indexes and not for moderate or fast ones; an explicit
doc_candidates argument, or a doc_candidates setting the user set,
overrides the default either way in every mode.

• pipeline: the switch is unset/on/off; unset means on only when the
  effective mode is full.
• MCP: forwards only an explicit choice (argument, then a user-set config
  value) to daemon jobs and workers; daemon auto-index and watcher/UI jobs
  now carry a user-set value too (their workers have no config, so
  doc_candidates false was ignored there).
• fast + doc_candidates on runs the semantic pass's corpus phases and the
  doc phase only: candidates, and still no SIMILAR_TO or
  SEMANTICALLY_RELATED edges.
• config: doc_candidates joins the config key table (default auto);
  config set doc_candidates ... was refused as an unknown key before.
  README, docs/CONFIGURATION.md and the MCP schema describe the default.

Proof (macOS arm64, django fb11376):
full 11,304 rows · moderate 0 · fast 0 · full+off 0 · moderate+on 1 ·
fast+on 1 (moderate/fast discovery skips docs/) -- nodes and edges are
identical between on and off in every mode; fast+on adds 0 semantic
edges.

• New tests (RED on the old code): pipeline
  pipeline_doc_candidates_default_follows_mode,
  pipeline_doc_candidates_explicit_wins_over_mode,
  pipeline_doc_candidates_fast_on_legacy_incremental; mcp
  tool_index_repository_doc_candidates_follow_mode,
  tool_index_repository_doc_candidates_forwarded_only_when_explicit;
  daemon_application
  daemon_application_programmatic_index_forwards_set_doc_candidates.
• mcp 374 passed (4 Windows-only skips), daemon_application 73 passed,
  pipeline/store_arch/cli/runtime_settings/index_policy/pipeline_frozen
  green on the release tree.