fix(coding-agents): drop harness transport wrappers from retained turns (#3023)

[nicoloboschi]

Closes #3023.

What I found

The issue names two cases. Measured against 400 real Claude Code transcripts on this machine, both are already handled here — and a third one, which it doesn't name, is live:

#3023 claim	in coding-agents	why
skill bodies retained as user conversation	no	Claude Code marks them isMeta: true (8/8 in the sample); the reader drops isMeta lines
<system-reminder> retained	no	they arrive inside tool_result blocks, which the reader drops entirely
<task-notification> retained	YES	39 in 400 transcripts, delivered as plain type:"user", isMeta:false, string body

So the harness's background-task plumbing — task id, tool-use id, status, summary — was being fed to fact extraction as things the user said.

Reproduction

Running the real readClaudeTranscript over a real transcript, before:

retained turns: 243
turns that are a harness task-notification: 1   (519 chars, role "user")
"<task-notification>\n<task-id>b46ca19nr</task-id>\n<tool-use-id>toolu_0127…</tool-use-id>\n<status>stopped</status>…"

After:

before: 243 turns, 1 harness-noise turn
after:  242 turns, 0 harness-noise turns
real turns preserved: true

The fix

One tag added to MEMORY_TAG_RE, which already covered codex's <hook_prompt> for exactly this reason — its comment calls them "the host's own hook-transport wrappers… transport noise, not the user's work". <task-notification> is that category for Claude Code.

<system-reminder> joins it as well. Not reachable today (dropped tool_result), but it is the same class and the rule is tag-structural rather than content-guessing, so listing it costs nothing and survives the harness moving where it puts them.

Stripping removes the block and keeps surrounding text: a message that is nothing but a wrapper renders empty and is dropped as a no-content turn, while a real message that merely mentions one keeps the user's words. That is deliberately the opposite of the old plugin's unanchored strip_channel_envelope, which replaced the whole message with the tag's inner text (#3124).

Test

433 passed, 19 skipped. tsc --noEmit and ./scripts/hooks/lint.sh clean.

Three new cases: a notification-only message producing no turn at all; a message with a wrapper embedded in real text keeping the text; and a <system-reminder> delivered as user text being stripped.