I14 [CLI]: API ergonomics tail — I37 template error, I53 verbose headers, I54 topic dedupe

[espadonne]

Summary

Final I-campaign bundle before v0.x.0 cut. Closes 6 findings (2 MED + 3 MIN + 1 PERF):

• I37 (MED): api -t '{{.nonexistent}}' now prints api -t: unknown response field "nonexistent" instead of leaking the template: api:1:2: executing "api" at <X>: position prefix.
• I45 (MIN): no-op — covered by I9 server-side strict boolean parsing.
• I53 (MED): api --verbose echoes the headers the api.Client actually injects (User-Agent, Accept, Content-Type, Authorization redacted), not just -H overrides.
• I54 (MIN): pinned add-topic idempotency in mergeTopics — duplicate adds + casing-shifted re-adds collapse to a single entry.
• I55 (PERF): deferred with rationale (gh-compat doesn't emit totalCount on list-array commands either; nextPage needs server cooperation across every list endpoint).
• I56 (MIN): process debt — leftover audit-repo cleanup tracked in the I14 doc.

See .docs/audits/I14-api-ergonomics-tail.md for the full closeout.

Commits

• eeca603 — Client.VerboseRequestHeaders helper (I53 prereq)
• d2b35cb — I37 + I53 wiring in pkg/cmd/api
• feed1de — I54 regression test

Test plan

• go test ./pkg/cmd/api/...
• go test ./pkg/cmd/repo/edit/...
• go test ./internal/api/...
• go test ./... (full suite)
• CI: lint + vet + multi-platform