feat(session-text): paste helper that delivers text to any focused pane, including raw terminals #830

Motivation

Prompt templates must work in rendered agent panes, agent terminal views, and raw terminal panes running unsupported harnesses. The API Key Vault in #831 uses the same insertion path.

Implemented Contract

• Shared renderer text delivery preserves template append/replace semantics and never submits.
• Terminal insertion routes through the mounted xterm owner, not an unguarded raw write.
• Respect current visibility, attach/replay state, and bracketed-paste mode. Single-line text works without that mode; multiline and unsafe terminal-control input are refused when they cannot be pasted safely.
• Preserve the captured session and picker lifetime through asynchronous preparation and wake. Refused writes keep the picker open without automatic retry into a replacement backend.
• Plain key insertion appends without template paragraph spacing.

Acceptance Criteria

• Rendered panes receive editable unsent drafts.
• Raw terminals and agent terminal views are valid template targets.
• Mounted-owner and paste-mode checks prevent unsafe or stale delivery.
• Deferred-wake cancellation, rejected writes, legacy kind normalization and control-byte refusal have regression coverage.
• Typecheck and focused unit/renderer tests pass.
• PR CI and manual terminal/authentication smoke verification.

Verification

Related branch: feat/api-key-vault. Claude and Codex review fixes are included. Latest focused set: 89 passing tests across 12 files. Desktop/remote build passes. A low-concurrency full-suite run exceeded ten minutes with unrelated lazy-Markdown and fixture failures; this is not recorded as a green full-suite gate.