Mark the leaking turn in the timeline (#336)

[mando]

Stacked on #332 (mando-issue-327-secret-scanning) — retarget to main once that merges.

The credential warning from #327 says what leaked but not where, so finding it means scrolling
the whole session. The findings already carry interactionSeq and chunkType; this uses them.

What changed

The timeline marks the leaking turn. A turn where the scanner found a likely credential gets a
red shield chip naming the category and the redacted hint, on the prompt half or the response half
it actually matched. The card carrying a finding also gets a tinted edge so it's spottable while
scrolling.

The banner deep-links to the turn. Each finding in SecretFindingsBanner is now a button that
opens the Timeline tab focused on that interaction and outlines the card. Findings read
"Interaction 4: AWS access key (AKIA…WXYZ) in your prompt", and the timeline's details rail now
numbers each interaction so that reference points at something visible. The list still stops at
five, and the overflow line now says the rest are marked on their turns.

No new endpoint. /api/session/:id already returns secretFindings, so SessionDetail passes
them to SessionTimeline as a prop. Nothing new crosses the wire, and findings stay local-only.

Three behaviors worth calling out

• Markers don't need retained text. With retainText off the timeline shows no prompt or
  response body, and the marker still says which turn and which kind of credential. That's the part
  the user acts on.
• A marker shows the first place a credential appeared, not every place. The scanner dedupes
  across the whole session, so a key pasted once and echoed in three later replies is one finding at
  its first location. Unchanged from Scan session content for exposed secrets/API keys and surface findings to the user #327, but it's now visible, so it's documented.
• Markers survive dismissal. Dismissing silences the banner ("I know about this"); the marker is
  an annotation on a turn the user navigated to on purpose, so it stays.

Implementation

The focus mechanism used to mean "a task chapter"; it now carries a kind, so a request can name a
task (the task list's timeline links, unchanged) or a single interaction (a finding).

A target that isn't in the timeline scrolls nowhere rather than somewhere arbitrary, and says so.
That case is narrower than it sounds: findings and the interaction spine are written from the same
array in one transaction, so the store can't disagree with itself. But the banner and the timeline
are two separate fetches, so indexing a session again under an open tab can leave a finding pointing
at an interaction the newer timeline data no longer has. Without the note the click switched tabs and
highlighted nothing, and with several tasks the timeline opens collapsed, so there was nothing to see
at all.

Two pure modules come out of the components so the logic is testable without a DOM:

• web/src/lib/secret-findings.ts — category labels, display order, the per-interaction grouping the
  timeline marks turns with. The banner's copies of the labels and the ordering move here.
• web/src/lib/timeline.ts — chapter grouping (lifted out of SessionTimeline unchanged) plus focus
  resolution.

Not done

The issue's optional enhancement (re-scan text at serve time to return transient match offsets for
in-place highlighting, SearchSnippet-style) is left out. It needs a serve-side re-scan and a
careful "offsets in, secrets out" contract, and the marker answers "which turn and what kind"
without it. Worth its own issue if we want the highlight.

Verification

• bun test — 788 pass, 0 fail (32 new: finding labels/order/grouping, chapter grouping, focus
  resolution including the stale-target and no-request cases, and the stale-link note).
• bun run typecheck and bun run build:web clean.
• Ran serve against a throwaway store seeded with demo data plus two obviously-fake credentials
  (the same non-functional constructions the scanner's unit tests use) and confirmed
  /api/session/:id carries the findings through to the session payload the timeline reads.
• Not verified: the rendered marker in a browser. No agent-browser on this machine, so the chip,
  the outline and the scroll are unphotographed — worth a look before merge.

Design notes updated in docs/internals/secret-scanning.md.

Closes #336