fix(lint): preserve Obsidian heading, block, and embed link syntax

[Aldominguez12]

Problem

The wikilink regex in openkb/lint.py captured the whole [[...]] body as a single page target. Valid Obsidian syntax therefore never matched a known target:

• [[page#Heading]] / [[page#^block]] — heading and block links
• ![[file.png]] / [[report.pdf]] — attachment embeds and links
• [[#Heading]] — same-page fragment links
• ![[concepts/x]] — note embeds

Consequences:

• openkb lint --fix destroyed valid links: strip_ghost_wikilinks demoted them to plain text. A wiki-wide sweep silently mangled hand-written notes in explorations/ — a data-loss path.
• openkb lint reported them all as broken links (false positives).
• Note embeds created no graph edges in visualize and could produce false orphans.

Fix

• Parse the embed marker, target, fragment, and alias as named regex groups.
• Validate only the page target; fragments survive fuzzy canonical rewrites ([[concepts/Gist_Memory#Notes]] → [[concepts/gist-memory#Notes]]).
• Pass through attachment embeds/links (extension whitelist) and same-page [[#Heading]] links untouched.
• Treat note embeds ![[concepts/x]] as regular page links: validated, rewritten keeping the !, counted as incoming links for orphan detection, and picked up as graph edges by visualize (which reuses _extract_wikilinks).

Verification

• New TestObsidianSyntax coverage: heading/block/fragment preservation through direct and fuzzy matches, ghost demotion without ! residue, attachment/note embed handling, and a regression test asserting a hand-written explorations/ note survives a wiki-wide lint --fix byte-for-byte.
• pytest tests/test_lint.py tests/test_lint_cli.py tests/test_visualize.py tests/test_chat_session.py tests/test_remove.py green; ruff check/format and mypy openkb/lint.py clean.

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