Fix empty transcript on resume; add shift+↑/↓ prompt jump

[tdwd]

The bug

Resuming a session showed no chat history for any project whose path contains a space, dot, or underscore.

projectSlug replaced only / with -. Claude replaces every non-alphanumeric character — one dash each, no run-collapsing:

cwd	claude writes	cathode looked for
/Users/w/Work/Triple Down/web	-Users-w-Work-Triple-Down-web	-Users-w-Work-Triple Down-web ❌
/Users/w/.config/wezterm	-Users-w--config-wezterm	-Users-w-.config-wezterm ❌
/Users/w/Work/x/fuel_forecaster	...-fuel-forecaster	...-fuel_forecaster ❌

The lookup landed on a path that doesn't exist, and both readers degrade silently to empty: the ctrl+r picker lost claude's own sessions (leaving only cathode's own store — which is why resume still worked, just with a blank transcript), and the replay rendered nothing. Projects without special characters, including this repo, were unaffected — hence the intermittency. Nine of 47 local project dirs were hit, including every .claude-worktrees worktree via the leading dot.

Second cause, same symptom

claudeRecord typed message.content as an array, but claude also writes it as a bare string for a plain typed prompt. That decode fails, and loadPriorTranscript skipped the whole record on any error — dropping the user's own prompts out of a replay. claude_sessions.go already handled both encodings in its own decoder; the two had diverged. They now share contentBlocks.

Changes

• projectdir.go (new) — the corrected slug rule plus claudeProjectDir, now the single resolution point for the picker and the replay. When the slug misses, it falls back to matching the cwd stamped inside the session records — ground truth, so resume survives future drift in claude's naming instead of failing silently. The scan stays off the common path.
• transcript.go — routes through claudeProjectDir; contentBlocks decodes both content encodings.
• claude_sessions.go — drops its duplicate decoder and the moved helper.
• CLAUDE.md — documents the rule and why getting it wrong is silent.

Also lands jump.go: shift+↑/↓ steps the viewport through past prompts, deriving the anchor from vp.YOffset (via m.entryLine) so it composes with wheel, PageUp and auto-follow rather than holding a cursor to invalidate.

Testing

go build, go vet, go test ./... all pass. New tests pin the slug rule against real observed directories, the recorded-cwd fallback, slug-wins-over-scan, both content encodings, and an end-to-end replay from a spaced cwd.

Verified live against the previously-broken session: the picker now finds it (opus-5, first prompt intact) and the replay returns 40 entries with the context gauge seeded at 820,451 tokens. Before the fix, both were empty.