feat: default WebM export to recorded wall-clock timing

[ThomasK33]

What changed and why

WebM exports defaulted to accelerated replay timing, which clamps every idle gap to 100ms. Sessions dominated by idle time (most interactive sessions) rendered as flash-by videos ~2s long, making recordings hard to review.

• Default timingMode is now recorded — exported videos play at the recorded wall-clock pace. --timing accelerated and --timing max-speed remain available as explicit opt-ins.
• Accelerated mode retuned for watchability — idle gaps now clamp to 400ms (was 100ms) and each frame holds at least 100ms (was 50ms), so opting into time compression no longer produces a flickery strobe.
• Updated --timing help text and docs/USAGE.md.

Automation-facing impact

The CLI JSON envelope shape is unchanged. Only the default value of metadata.timingMode differs (recorded instead of accelerated) when --timing is omitted, and accelerated exports are proportionally longer due to the retuned clamps.

Validation

• mise run ci — full gate green (150 test files, 1512 tests, build + install smoke)
• Targeted: test/unit/export/webm.test.ts, test/unit/commands/record-export.test.ts, test/integration/record-export.test.ts, test/e2e/export-fixtures.test.ts

Dogfood proof

Real session in an isolated AGENT_TTY_HOME running echo first; sleep 2; echo second; sleep 2; echo third (~4s of wall time in sleeps), exported via record export --format webm:

Export	metadata.timingMode	Video duration (ffprobe)
No flag (new default)	recorded	7.20s — matches session wall time + 1s final hold
--timing accelerated (new 400ms clamp)	accelerated	4.00s
--timing accelerated (old 100ms clamp)	accelerated	2.84s — the old flash-by behavior

Last-frame screenshot of the recorded-default export confirmed correct rendered content (including the prompt's own took 4s).

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Generated with mux • Model: anthropic:claude-fable-5 • Thinking: high