feat: support selective sidecar pruning

[steipete]

Summary

• add an optional sidecar prune predicate
• preserve default prune-all behavior when no predicate is supplied
• cover generated-Markdown pruning that leaves unrelated share files intact

Proof

• GOWORK=off go test -count=1 ./...
• GOWORK=off go vet ./...
• autoreview: no accepted/actionable findings
• Notcrawl share regression suite passes against this branch

No release or tag requested.

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: feat: support selective sidecar pruning This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.

[steipete]

Landed as 3f71740424ec8b6bdcc44f8a91194e7e9e943d21.

Proof before merge: full tests and vet green; autoreview clean; Linux, Windows, CodeQL, and secret-scanning checks green; Notcrawl share regression suite passes against the selective prune API.

Post-merge checkout: main, clean and synchronized with origin/main. No release or tag created.