feat(kb): vk kb list/delete/prune for 0.6.0 — clean up the 425-kb backlog

[voidkey]

Summary

vk kb list / delete / prune — manage vectoria knowledgebases. Headline command is vk kb prune, designed to clean up the 425 accumulated kbs from vk create's one-kb-per-invocation pattern (0.5.2 closed the future leak; this PR adds the bulk-cleanup tool).

The user can now run, post-merge:

```bash
vk kb prune --pattern 'vibeknow-cli-*' --yes # actually delete CLI's own orphans
```

Safety contract (load-bearing, all integration-test-pinned)

• Refuses to run without --pattern or --older-than (exit 2). No "delete everything" shortcut.
• Dry-run by default: prints matched kbs without DELETE. --yes (or VIBEKNOW_ASSUME_YES=1) actually deletes.
• Idempotent 404: counts as success (rm -f semantics).
• Exit codes: 0 success / 2 bad input / 5 all-fail / 7 partial-success.

Diff (10 commits)

• 0f1015f feat(vectoria): add ListKBs client method — paginated list wrapping GET /v1/knowledgebases?offset=N&limit=N
• 772610e feat(durfmt): add ParseAge supporting Nd day-suffix shortcut — new internal/durfmt pkg parsing 7d / 24h / 1h30m
• 6ac258a feat(kb): add subcommand skeleton + i18n keys — 17 keys in en+zh
• 8d742ab feat(kb): vk kb list with --pattern and --older-than filters — client-side glob + age filter; text + JSON output
• e309b83 feat(kb): vk kb delete with idempotent 404 handling
• d558dbc feat(kb): vk kb prune with dry-run default, pattern/age filters, partial-success exit 7
• 9d3b8a2 test(integration): kb prune dry-run-by-default, apply, filter-required — 3 end-to-end tests against mock vectoria
• 9516ce4 docs: document vk kb subcommand family for 0.6.0 — CHANGELOG / AGENTS / README + zh
• 6586d69 chore: bump version to 0.6.0
• ba44016 refactor(kb): final-review fixes — exit-2 consistency, pre-flight, comment

Test plan

• go test ./... passes (incl. 3 new integration tests for kb prune + 4 filter unit tests + 2 prune-validation unit tests + ListKBs unit test + ParseAge unit tests)
• go build ./... clean
• vk kb --help lists three subcommands (list, delete, prune)
• vk version reflects 0.6.0
• vk kb prune without filter → exit 2 with clear message
• vk kb prune --pattern 'bogus-*' → real-backend smoke walked 5 pages, 0 matches, exit 0 (proves pagination + filter wiring against real vectoria)
• Manual smoke (post-merge): run vk kb prune --pattern 'vibeknow-cli-*' --yes on the user's tenant; expect ~10-20 orphan kbs to be cleaned. Verify count drops via vk kb list --output json | jq .total.

Final review status

🟡 APPROVED with minor follow-ups (opus-level review). 3 of 4 important findings applied in commit ba44016. Skipped:

• page/size client-side clamping — consistent with existing vk video list; backend already returns clear error on bad values
• locale pinning in integration tests — cross-cutting concern, separate follow-up
• fmt.Println vs cmd.OutOrStdout() — pre-existing pattern across the codebase
• 0d edge case in ParseAge — documented behavior (interpreted as "no age filter")

Out of scope

• vk kb show <id> — vk kb list --pattern <exact-name> covers the common lookup; backend has no per-id GET anyway (returns 405).
• Doc count column in vk kb list — N+1 fetches; can add --with-doc-count later if asked.
• Auto-cleanup on each vk create — surprising side-effect; users invoke vk kb prune explicitly.

Spec / plan

• Spec: docs/superpowers/specs/2026-05-14-kb-subcommands-design.md
• Plan: docs/superpowers/plans/2026-05-14-kb-subcommands.md

(Both live in the untracked docs/ per team convention; not in this PR.)