feat(sessions): Copy as Markdown — export a session, scrubbed - #65

The M4 export slice (levelcode-sessions-experience.md §6): one action turns a
session into a clean Markdown transcript for pasting into a PR, and it is the
seed of a later share-a-run.

**It scrubs, and that is not incidental.** chat-sessions-design.md §10 already
decided this: transcripts at rest are the same trust class as your code, but
"anything that later *shares* a session must scrub — that is that feature's
burden." Export is the first surface that shares one, so redactSecrets — built
last week for project memory — is passed in at the call site rather than baked
into the renderer, so the scrub is visible where it happens instead of being a
property you have to know the module has.

**Roles are bold labels, never headings.** A turn routinely contains `## …` and
fenced code; a heading-based role label is visually outranked by the content it
is supposed to delimit, and `### LevelCode` above a reply opening with `#` reads
as though the model wrote the section title. Bold plus a rule survives every
renderer at every nesting depth.

The success message counts TURNS, not characters: "Copied 14 turns" tells you
whether you got the session you meant.

**The fifth button.** The action row is two fixed-height lines and
`flex-wrap: nowrap`, so buttons that do not fit do not wrap — they overflow.
Five labelled buttons need ~300px and a sidebar is routinely narrower, so the
card is now a container and the labels collapse below 360px, leaving icons at
~184px. Every button already carried title + aria-label, so nothing is lost to a
pointer or a screen reader — only to the eye, and only when there is no room.

Both webviews carry the card, and the pure block between the SESSIONS-PURE
markers must stay byte-identical across them — sessionsView.html is synced, which
is the existing test catching exactly what it was written for.

Tests: 5 new in sessionEvents.test.js (structure, heading safety, the scrub, the
opt-in, degenerate sessions), 2 in webviewCss.test.js pinning the overflow guard
in BOTH sheets, and the action-row test updated to five buttons. The CSS guard is
pinned because the failure is invisible in a wide window: whoever adds a sixth
button will not see it break — a user with a narrow sidebar will.

Verified non-vacuous: dropping the container query 13/15, the container-type
13/15, the aria-label 14/15, the export scrub 10/13. All 32 suites green.

…ename

Two review findings on #65:

- toMarkdown trimmed each turn's text, which changes Markdown semantics: a
  leading 4-space indent (indented code block) got de-indented and a trailing
  "  " (hard line break) was eaten. Turn text is now emitted verbatim aside from
  redaction — the `**role**` blank line already supplies the blank an indented
  block needs. Pinned by a new test (indented code + trailing hard-break survive).
- The default save-dialog filename was derived from the RAW entry.title, not the
  scrubbed copy toMarkdown makes — and the filesystem sanitiser only strips
  slashes/colons, so a credential's own characters survive it (ghp_ABC… →
  ghp-abc…). entry.title now goes through redactSecrets before the stem, so a
  token in a title can't leak into the save path. Pinned by a source invariant.

32 suites green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>