feat: add ccs prune to losslessly shrink bloated conversations

[brtkwr]

What

A ccs prune subcommand that shrinks large conversation files by removing only data that duplicates content kept elsewhere, so pruned conversations still resume with their full dialogue.

Investigating real files showed the referenced pruning blog post targets a message shape that no longer exists (no agent_progress/bash_progress lines), and that the genuinely-duplicated data is different:

• toolUseResult - byte-for-byte a copy of the tool_result already inside message.content (verified: 189,154 vs 189,156 bytes on the biggest tool line). The model resumes from message.content, so dropping this loses nothing it reads.
• file-history-snapshot lines - rewind/checkpoint backups, redundant across snapshots and with the files on disk. Pruning loses rewind history, not the conversation.

Across the 8 largest files these are ~25% of bytes; everything else (assistant/user messages, attachments) is real content and left untouched.

Safety

• Dry run by default - ccs prune only previews; --apply is required to rewrite anything.
• User and assistant message lines are never modified or dropped.
• Each file is streamed to <path>.pruned and atomically renamed only if the conversation line count is unchanged - a failed or partial prune never clobbers the original.
• Default skips files under 50MB; --apply confirms before writing (-y to skip).

CLI

ccs prune                       # dry-run preview (files >= 50MB), no changes
ccs prune --apply               # prune after confirmation
ccs prune --apply --min-size=200  # only files >= 200MB
ccs prune --apply --no-tool-results  # keep tool results, drop only snapshots
ccs prune --apply -y            # skip the prompt

Read-only default run on real data:

  ...infra/4540c603-....jsonl     75MB ->     53MB  (-21MB)
  ...auth-gateway/b8d9d564-....jsonl  61MB ->  44MB  (-16MB)
  ...
Would reclaim 69MB across 5 files. Re-run with --apply to prune.

Tests

• TestPruneStreamRemovesDuplicatesKeepsDialogue - snapshot dropped, toolUseResult stripped, message.content/dialogue preserved, conv-line count invariant, output stays valid JSON.
• TestPruneOptsRespected - --no-snapshots / --no-tool-results honoured.
• TestPruneFileReplacesAndShrinks - atomic replace, file shrinks, .pruned temp cleaned up, integrity preserved.
• TestPruneFileDryRunLeavesFileUnchanged - dry run reports savings but doesn't touch the file.
• go test -cover = 63.9% (uncovered part is the runPrune CLI driver; the data-safety core is covered).

Note

CLI half only. The TUI per-conversation prune action (a key + confirmation, like Ctrl+D delete) is a planned follow-up. Resume-safety of dropping toolUseResult is provable for the model's view but not for Claude Code's own UI/rewind, so try --apply on a low-risk conversation first.