[#13] Optional: render topics as Obsidian #hashtags

[VGonPa]

Closes #13.

Summary

Adds an opt-in config flag, [output] topic_style = "wikilink" | "hashtag", so
the in-body **Topics:** line on each item note can be rendered as Obsidian
hashtags instead of wikilinks. Default is wikilink — fully
backwards-compatible.

• wikilink (default): **Topics:** [[ai-coding]] · [[software-engineering]]
• hashtag: **Topics:** #ai-coding #software-engineering

Frontmatter tags:, the _index.md ## Topics ranking and the topic-page
post lists are unchanged — those are navigational by design and stay
wikilinks regardless of topic_style.

Decisions (resolved in the PRD)

1. Replace, not alongside. Hashtags already navigate when a same-named
   note exists (every topic page is topics/<slug>.md); "alongside" doubled
   the line noise without paying for itself.
2. Flag name: [output] topic_style — sits next to the existing
   [output] language. Backwards-compatible default "wikilink".
3. Slug compatibility: kebab-case slugs (^[a-z0-9]+(?:-[a-z0-9]+)*$,
   per models.py:Topic.slug) are a strict subset of Obsidian's tag
   charset [A-Za-z0-9/_-], so no slug change is needed.
4. Frontmatter: unchanged in both modes (the orthogonality invariant is
   asserted in test_generate_renders_topics_as_hashtags_when_requested).

Implementation

• Config.topic_style: str (src/xbrain/config.py) — read from
  [output].topic_style, defaults to "wikilink", validated against
  SUPPORTED_TOPIC_STYLES = ("wikilink", "hashtag"). Unknown value →
  clean ValueError listing the supported set.
• generate(..., topic_style: str = "wikilink") — threads the value
  through to _enrichment_lines. Defensive ValueError at the entry
  point for programmatic callers (mirrors how output_language is
  validated by strings_for() on entry).
• cli.py:_run_generate passes cfg.topic_style through.
• config.toml.example and the README config table + TOML block document
  the new key. README Layer 1 gets a one-sentence note pointing to the
  hashtag mode near the existing example.
• No new module — topic_style is a layout choice, not a locale concern,
  so it stays on Config rather than entering xbrain.i18n.

Tests

7 new tests, all green:

• tests/test_generate.py
  • test_generate_renders_topics_as_wikilinks_by_default — default mode
    keeps the exact current line.
  • test_generate_renders_topics_as_hashtags_when_requested — same
    item, hashtag mode, asserts the body line + frontmatter unchanged.
  • test_generate_hashtag_mode_does_not_affect_index_or_topic_page_lists
    — the ## Topics ranking still uses wikilinks.
  • test_generate_rejects_unknown_topic_style — ValueError listing
    supported values.
• tests/test_config.py
  • test_load_config_defaults_topic_style_to_wikilink
  • test_load_config_round_trips_hashtag_topic_style
  • test_load_config_rejects_unknown_topic_style

Quality gate (uv run poe check)

✅ Ruff (linting)       PASS
✅ Ruff (format)        PASS
✅ Mypy (types)         PASS
⚠️  Radon (complexity)   WARN - Has grade C   (pre-existing, not introduced by this PR)
✅ Bandit (security)    PASS
✅ Vulture (dead code)  PASS
✅ Interrogate (docs)   PASS - 73.5%
✅ Detect-secrets       PASS
✅ Tests                PASS - 306 tests
✅ Coverage             88%
✅ Deptry (deps)        PASS
ALL CRITICAL CHECKS PASSED

Out of scope

• CLI flag --topic-style for one-off overrides (file a follow-up issue if
  needed).
• Switching the default from wikilink to hashtag.
• Changing topic-page or index rendering.