docs: restore task composer keyboard guidance (#19680)

## Thinking Path

> - Paperclip helps people manage AI agents and tasks.
> - Task composers have two keyboard modes.
> - The product guide explains these modes.
> - The routine feature merge removed that paragraph.
> - This pull request restores the paragraph beside the routine
guidance.
> - Readers can find both behaviors.

## Linked Issues or Issue Description

Refs #19128. This change addresses [the late documentation
review](https://github.com/paperclipai/paperclip/pull/19128#discussion_r4492144137).

**Issue type**

Missing documentation.

**Where is the issue?**

The Task Management section in doc/PRODUCT.md.

**What's wrong?**

The routine feature merge removed the paragraph about Enter,
Shift+Enter, and the optional Command/Ctrl+Enter setting.

**Suggested fix**

Restore the exact paragraph from before the merge. Keep the routine
section.

## What Changed

- Restore five lines in doc/PRODUCT.md.
- Keep the routine responsibility section.

## Verification

- Final qualification at `c423853f51c2bb698a65c746ed5e32d084ef9046`:
[required CI
passed](https://github.com/paperclipai/paperclip/actions/runs/42317059645/job/128236866986),
including typecheck, build, all test lanes, E2E, and verify. The
snapshot has 58 successful check runs, three conditional skips, one
neutral advisory job, and a successful legacy Snyk status. No pending or
failed checks remain. Greptile and Paperclip Review both score this
exact head 5/5; all review threads are resolved.

- A local content check confirms that the restored paragraph matches the
text before #19128. Current keyboard handling and the Experimental
setting still match the paragraph. Whitespace checks pass.
- Local `pnpm verify` could not start because the restored checkout
lacked its `tsx` dependency. The dependency-free selection command
completed with exit 2 because PRODUCT.md is a global input. Neither
attempt is represented as a test pass; the successful final CI supplies
that broader coverage.
- This restores documentation only. Runtime code, schema, and behavior
do not change. Review verified the source behavior and the removed
paragraph; no separate browser acceptance is claimed.

## Risks

- Low risk. The change restores existing product guidance.
- No runtime behavior or database change is included.

## Model Used

OpenAI Codex, based on GPT-6, with reasoning, tool use, and code
execution. The exact runtime model ID and context window are not exposed
in this session.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have searched GitHub for duplicate or related PRs and linked
them above
- [x] I have either (a) linked existing issues with `Fixes: #` / `Closes
#` / `Refs #` OR (b) described the issue in-PR following the relevant
issue template
- [x] I have not referenced internal/instance-local Paperclip issues or
links (only public GitHub `#NNN` / `github.com/paperclipai/paperclip`
URLs)
- [x] My branch name describes the change (e.g. `docs/...`, `fix/...`)
and contains no internal Paperclip ticket id or instance-derived details
- [x] I have run tests locally and they pass (content and whitespace
checks pass; runtime tests do not apply to this text restoration)
- [x] I have added or updated tests where applicable (no runtime
behavior change)
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] All Paperclip CI gates are green
- [x] Greptile is 5/5 with no open P2s, recommendations, or follow-ups
- [x] I will address all Greptile and reviewer comments before
requesting merge

Co-authored-by: Paperclip <[email protected]>

doc/PRODUCT.md

@@ -147,6 +147,11 @@ the latest agent that assigned it to them. If no assignment is available, it use
 the latest other commenter, or leaves the recipient empty. A manual recipient
 choice stays selected while task data refreshes.
 
+Task reply and new-task composers submit with Enter by default; Shift+Enter
+adds a new line. Settings → Experimental → **Command/Ctrl+Enter to Submit**
+restores the previous behavior, where Enter adds a new line and Command/Ctrl+Enter
+submits.
+
 ### Routine responsibility
 
 The routine Overview shows its responsible user beside Default agent. People with