feat(index): index .mdx files as markdown-twin content nodes

[germanamz]

What

Adds .mdx file support to Tusk. An .mdx file is a full markdown twin: indexed, queryable, renderable, sub-unit-split, editable via the node/edge write tools, and a valid edge source/target — parsed as markdown (frontmatter + prose + wikilinks), exactly like .md.

Why

MDX is markdown-with-JSX; vaults that mix .md and .mdx notes were silently dropping every .mdx file (the reindex walk only picked up .md/.html/.htm).

How

• .mdx is indexable — added to index.indexableExts.
• Id retains its extension (foo.mdx) so foo.md and foo.mdx never collide on the nodes.id primary key (Decision feat: add circular parent detection (Phase 1) #12). .md stays the only bare-stem kind.
• One id rule + its inverse. The three previously-divergent id-derivation sites (reindex.nodeIDForPath, node.nodeIDForPath, parse.go) now delegate to a single index.NodeIDForPath (strip only .md, retain every other indexable kind). Its inverse index.FilePathForID replaces every hardcoded sourceID + ".md" reconstruction (edgewrite.go ×5, doctor.go). FilePathForID treats a .md-suffixed id as a bare stem (a *.md.md file) since .md is the only stripped kind, so it's a true inverse.
• Content routing needs no new branch — .mdx falls through the markdown default in ParseContentFile / NodeText / the sub-unit split. JSX tags and import lines parse as markdown (raw HTML / paragraph text). No MDX compiler — documented limitation.

Verification

Automated: full suite (go test ./...) + -race on index/node/reindex green; gofmt/vet/golangci-lint clean. New tests: NodeIDForPath/FilePathForID round-trip incl. the .md.md case; md+mdx coexistence through the real reindex.Run pipeline; Create-accepts-.mdx; rename referrer-retarget to a moved .mdx node (the #687 analogue).

Live e2e (built binary, scratch vault):

• .mdx indexed as a first-class note; foo.md + foo.mdx coexist as distinct nodes
• body [[wikilink]] materialized as an edge; full sub-unit split
• node get / query / node modify / edge add / node move all work on the .mdx id; frontmatter rewrites preserve the MDX body (imports + <Callout> JSX)
• referrer edge retargeted on move; doctor clean; double-reindex converges to 0 indexed / 0 skipped
• regression fix confirmed live: edge add on a changelog.md.md node writes to the correct file

Review

An adversarial multi-agent review of the diff surfaced one real (minor) regression — FilePathForID mis-reconstructing a *.md.md file — which is fixed here, plus the durability tests above.

Out of scope

MDX/JSX compilation; .markdown extension; per-component semantic extraction.