Agent UX: pm append <id> "" is a silent no-op that still bumps updated_at (false freshness) — #588 empty-input fix didn't cover append #597

Summary

pm append <id> "" (empty / whitespace-only positional text) silently succeeds: exit 0, changed_fields: [], body unchanged — but updated_at is bumped to now, creating a false freshness signal. pm comments <id> "" correctly rejects empty text. This is the same empty-input silent-no-op class fixed for pm comment in #588/2026.7.19, but pm append was not covered — and this variant is worse because it mutates updated_at. Verified under CLI 2026.7.18.

Repro

pm update pm-REAL1 --body "test body"
# note updated_at
pm append pm-REAL1 ""            # exit 0, changed_fields: [], body still "test body"
# updated_at is now bumped to current time

pm comments pm-REAL1 ""          # Error: Command failed  <-- validated, as it should be

Observed: before updated_at: ...T14:17:59.455Z → after updated_at: ...T14:18:01.934Z, body unchanged, changed_fields: [].

Observed vs expected

• Observed: empty append is a no-op that still bumps updated_at; the updated_at bump directly contradicts changed_fields: [].
• Expected: reject empty/whitespace-only append text with a structured usage error (matching pm comments). At minimum, never bump updated_at when changed_fields is empty.

Why it matters for agents

An agent running pm append $id "$text" where $text resolves to empty (a very common templating outcome) sees exit 0 + a full item payload and concludes the append landed. Worse, the bumped updated_at makes the item look freshly modified, which pollutes pm activity, recency sorts, and pm next ranking (which weights updated_at) — so a no-op silently reorders the agent's work queue. Agents must learn per-command empty-input rules instead of relying on a uniform contract.

Duplicate check

Related to #588 (closed; pm comment --message with no text = silent no-op, fixed in 2026.7.19) but distinct: (a) different command (pm append), (b) the #588 fix did not extend to append, (c) this variant bumps updated_at (false freshness) which #588 did not. Confirms the empty-input validation contract in docs/AGENT_UX_CONTRACTS.md ("Input and tracker recovery") should be applied uniformly across text-accepting mutations.