feat(export): restore original nested paths on OKF re-export

[ethanj]

What

Re-exporting an OKF bundle that was imported from nested paths now restores each imported foreign document to its original location (e.g. tables/customers.md) instead of flattening it to concepts/<slug>.md, with the index and inter-document links pointing at those paths. This resolves the deferred follow-up from the OKF round-trip work — a bundle now survives import → export with its layout and links intact.

• Placement. A new resolver assigns each page its output path: native pages keep their slug-path; an imported page restores its x-okf.okfPath when that path is safe and uncontested, otherwise falls back to its slug-path with a warning. An ownership map guarantees a restored path can never displace another page.
• Links. The link resolver is path-aware, so a native page linking an imported doc ([[tables-customers]]) emits the doc's real path (/tables/customers.md); on re-import that link reverses back to a wikilink, so it round-trips with no drift. Foreign documents' own markdown links are preserved verbatim and now resolve, since the files are restored to where the links point.

Why

It makes llmwiki a faithful OKF pass-through: a third-party bundle keeps its directory structure and cross-document links across a round-trip, not just its contents and frontmatter.

Safety

Imported okfPath values are untrusted (editable page frontmatter), so the exporter is hardened: an unsafe or colliding path falls back to the slug-path with a structured warning; every write is realpath-confined to the bundle; and the output directory is handled carefully — the bundle's real target is resolved through symlinks before any directory is created and refused if it is the filesystem root, the project root, or inside .git; a non-empty directory that isn't a recognized OKF bundle is refused rather than clobbered; and a recognized prior bundle is wholesale-cleared (refusing if it contains a nested .git) so stale files don't linger. External output directories remain supported.

Test plan

• npx tsc --noEmit, npm run build, npm test (1663 passed), npx fallow (0 above threshold), dupes clean
• Nested round-trip: a foreign tables/customers.md + a sibling linking it survive import → export at their exact paths with the link intact, and re-import
• Native→foreign full cycle: [[tables-customers]] → /tables/customers.md → back to [[tables-customers]]
• Output-path resolution: ownership-map collisions (foreign-vs-native and foreign-vs-foreign), unsafe-path fallback with warning, exact-match reserved names (nested index.md allowed)
• Output-dir safety: refusals for filesystem/project root, inside .git (asserted never created), a symlinked parent resolving into .git, a nested .git in a bundle, and a non-empty non-bundle dir; symlinked-subdir cleanup and stale-file cleanup
• Backward compatibility: native-only bundles place every doc at <dir>/<slug>.md with unchanged TOC/links