feat(ui): parse file-path candidates with line and column suffixes

[sylwester-liljegren]

Issue

Re-split of #10340 per maintainer feedback (no separate tracking issue exists). Part 1 of 3.

Stack (review/merge in order): #11217 (this PR) → #11218 → #11219

Context

First slice of the clickable-file-links work from #10340, rebased onto current main and split into smaller reviewable pieces as requested. This PR only changes the path-parsing layer: turning inline code / link hrefs into structured { path, line, column } candidates that a later slice validates against the filesystem.

Implementation

• Replaces the regex parseFilePath (a "does this look like a file?" matcher) with two small helpers: extractSuffix (strips :line / :line:col / :start-end suffixes) and normalizeCandidatePath (adds a ./ prefix to bare relative paths, strips a/+b/ diff prefixes, leaves absolute/Windows/UNC paths alone).
• extractFilePathFromHref now returns { path, line, column } instead of a bare string, and preserves file:// decoding and Windows-drive handling.
• Both copies are updated: packages/ui/src/file-path.ts (shared opencode path, keeps its kilocode_change marker) and packages/kilo-ui/src/file-path.ts (Kilo-only).
• Detection intentionally shifts to "extract a candidate, let the filesystem confirm it" — confirmation lands in the dependent slices.

Screenshots / Video

N/A — no user-visible change in this slice.

How to Test

Manual/local verification

• N/A (pure parsing module; covered by unit tests below).

Reviewer test steps

1. From packages/ui/: bun test src/file-path.test.ts
2. Confirm 48 tests pass across extractSuffix, normalizeCandidatePath, and extractFilePathFromHref.

Blocked checks and substitute verification

• A standalone typecheck of just this slice fails on purpose: its consumers (marked.tsx, message-part.tsx) are updated in the dependent UI PR (feat(ui): parse and render clickable file links #11219). The full stack typechecks/tests/builds green. Substitute verification: ran this module's unit tests directly — agent-executed, 48 pass.

Checklist

• Issue linked above, or exception explained
• Tests/verification described
• Screenshots/video included for visual changes, or marked N/A
• Changeset considered for user-facing changes (the changeset lives in feat(ui): parse and render clickable file links #11219, which carries the user-facing behavior)
• I personally reviewed the diff and can explain the changes, including any AI-assisted work.

[sylwester-liljegren]

Closing in favor of a buildable 2-PR stack. The path-parsing change can't be its own PR: removing/altering the file-path API breaks its only consumers (marked.tsx, message-part.tsx) until the UI commit lands, so this PR's CI failed typecheck (marked.tsx importing the removed parseFilePath). Path-parsing is now combined with the UI in #11219, stacked on the independent filesystem-validation PR #11218. Both of those are green.