Add an instruction to link the plan/session in the PR - #85235

This documentation-only PR adds guidance for linking the plan and Copilot session when pausing work for approval.

Changes:

• Documents incomplete PR status and remaining work.
• Adds plan/session-linking guidance.

.github/copilot-instructions.md:126

• Could this define what counts as a usable session link and provide a fallback when the Copilot surface does not expose a stable, shareable URL? As written, this is mandatory, but an agent may only have a local session/plan.md and no URL it can put in GitHub; allowing the plan and approval request to be included inline when no shareable link exists avoids inaccessible or unverifiable links.

If a pull request already exists when work pauses for plan approval, update its description or add a comment that clearly states the implementation is **not complete**, summarizes what remains, and links to the Copilot session containing the plan and approval request so the user can provide further instructions.

.github/copilot-instructions.md:126

• This introduces a new repo-wide workflow pattern, but the existing documentation rule at lines 94-99 requires repo-wide patterns to be recorded in .github/memory/CONVENTIONS.md. Please add the session/PR-linking convention there as well, or explicitly document why this canonical instruction is intentionally the sole source, so agents that load the memory conventions do not miss it.

If a pull request already exists when work pauses for plan approval, update its description or add a comment that clearly states the implementation is **not complete**, summarizes what remains, and links to the Copilot session containing the plan and approval request so the user can provide further instructions.

.github/copilot-instructions.md:126

• Could this be made explicitly append-only? “Update its description” is ambiguous and could lead an agent to replace the author’s existing summary, acceptance criteria, or reviewer context while adding the handoff note. Please require appending without deleting existing content (or prefer a new comment) so the pause/approval handoff remains auditable.

If a pull request already exists when work pauses for plan approval, update its description or add a comment that clearly states the implementation is **not complete**, summarizes what remains, and links to the Copilot session containing the plan and approval request so the user can provide further instructions.