fix(viewer): percent-decode link targets in _extract_links

[oierreaemme]

Fixes #200

Problem

Per SPEC §5, links to file names containing spaces are percent-encoded (e.g. [report](weekly%20report.md)). _extract_links in okf/src/reference_agent/viewer/generator.py resolves the raw regex capture without decoding it, so the emitted edge id is notes/weekly%20report while the concept node id derived from the on-disk path is notes/weekly report. The edge never matches a node and is dropped: a bundle whose file names contain spaces renders with zero edges even though every link is spec-conformant.

Fix

Decode the captured target with urllib.parse.unquote before resolving it. Decoding happens before the scheme/absolute-path guards, so percent-encoded forms of external (https%3A%2F%2F…) or absolute (%2F…) targets are still skipped. Targets containing a literal % not followed by two hex digits are left untouched by unquote.

Scope

Deliberately minimal:

• absolute bundle-relative links (/Note.md) remain unsupported by the viewer — that is visualize: 0 edges for spec-recommended absolute links and CommonMark link titles #48 (PRs fix: viewer absolute links + spec alignment across codebase (fixes #48) #66/okf: fix visualize edges for absolute and titled cross-links #184);
• _LINK_RE still rejects literal unencoded spaces, which is consistent with markdown link syntax and with SPEC §5's encoding recommendation;
• the SPEC-side clarification of link encoding is tracked in Clarify if OKF links require URL encoding #112/okf: clarify URL encoding for cross-link targets (SPEC 5.4) #186.

Testing

okf/tests/test_viewer.py: 7 passed (6 existing + new regression test test_percent_encoded_links_become_edges, which fails without the fix).

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[oierreaemme]

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