chore: bump version to 0.5.0 + write 0.5.0 changelog

Bumps `;; Version:' header from 0.4.0 to 0.5.0 and AGENTS.md `Current
version' to match (it was still 0.3.0; missed at the 0.4.0 bump).

Per AGENTS.md versioning rules, minor (0.x.0) is the right step for
new features, and 0.5.0 collects:

- Typst inline equation preview (#5)
- Configurable Sixel encoder + per-invocation timeout (refs #4)
- Sixel inside tmux >= 3.4 (refs #2)
- Windows Terminal detection + GIF routing (closes #6)
- Inline mpv video playback on the Kitty backend
- Loud warning when the Sixel backend cannot find an encoder on PATH

Plus assorted fixes (ECH erase + pre-erase, heading folded-line
skip, Sixel parity, text sizing column guard / CPU / preview-mode),
the new `justfile' task runner, and documentation that hard-states
img2sixel as the recommended Sixel encoder.

CHANGELOG.org gets a new top-level section with the full feature
write-ups, bench numbers for the encoder swap, credits to @timfel
(encoder timeout pattern + Windows Terminal patch + ongoing Sixel
collaboration), and a reworded "what comes next" paragraph that
reflects the Sixel-in-tmux side as done while leaving Kitty APC
passthrough on the roadmap.

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