docs(lume): specify SSH stdin vertical slice

[injaneity]

Scope

Refs #1514. Refs #3624.

Design-only vertical slice: pipe/file → CLI → either existing SSH backend → guest stdin → EOF → exit status. The RFC proposes explicit CLI input ownership and --no-stdin, bounded NIO input, system-SSH parity, authentication separation, no-replay fallback, and local cancellation semantics.

The maintainer selected planning on #1514. The new CLI contract is proposed, not accepted. No product implementation is included.

Progress

• Revalidated lume ssh: piped stdin is not delivered to the remote command #1514 assignments/comments/timeline and active SSH work.
• Recorded selection on lume ssh: piped stdin is not delivered to the remote command #1514 and opened the contract discussion in [RFC]: Forward command stdin through lume ssh #3624.
• Created an isolated canonical-repository branch and a reviewable technical spec.
• Maintainer decision on default forwarding, --no-stdin, and no-input EOF.
• Product implementation and acceptance evidence after approval.

Boundaries and dependencies

PR #1701 owns binary output (#1513); PR #3272 owns event-loop lifetime. This proposal does not duplicate either contribution. Binary input is checked using guest-side length/digest oracles, not a binary stdout round trip. No desktop execution, MCP input feature, VM orchestration, or live output API. Internal concurrent output-pipe draining is included only to prevent full-duplex deadlock.

Validation

• Source/history review against main 25e578b.
• git diff --cached --check passed.
• Prettier 3.6.2 check passed for the spec.
• Relative repository links validated.
• Product tests, SSH fixtures, and VM smokes have not run: this PR changes one Markdown spec only.
• No VM was started; existing workspace modifications were left untouched.

Open design checks

Verify the pinned NIOSSH exec-acceptance event and cancellation-safe descriptor strategy before coding. Record macOS/Linux guest/backend evidence when implementing; a mock result is not transport proof. Full Cua Driver desktop certification is unrelated to this slice.

Thanks @pvulgaris for the original reproduction and diagnosis.