fix: add next-step hints to CLI errors and standardize empty states

[BunsDev]

Context

• Summary: UX audit cli: terse errors lack next-step hints; empty-state output is inconsistent #449 (follow-up to cli: help text is stale (harness list) and inconsistent across subcommands #448, stacked after fix: make CLI help text truthful and consistent #454 merged) found high-traffic error messages with no next step and inconsistent empty-state output. This PR standardizes both on the existing exemplars (no supported harness is available; run \coven doctor` for setup guidance, session `{id}` not found; run `coven sessions --all` ...`).
• Files changed: crates/coven-cli/src/main.rs, crates/coven-cli/src/pc/display.rs
• Closes cli: terse errors lack next-step hints; empty-state output is inconsistent #449

Implementation

• Approach: human-facing strings only; no control flow, exit codes, or --json bodies changed. The Cancelled no-Error:-prefix contract is untouched.
• Error sites upgraded (semicolon + next step, lowercase leaf voice):
  • prompt must not be empty → adds quote a task string, e.g. \coven run codex "fix the failing tests"`` (existing test asserts the stable prefix and still passes)
  • patch --non-interactive bails: issue-text bail now shows the full command shape; --harness bail names the flag values (codex/claude/copilot)
  • issue text must not be empty when provided → adds an example invocation
  • a response is required → type a non-empty answer, or press Ctrl-C to cancel
  • session {} not found in local store and summon's session \{id}` not found→ aligned with thecoven sessions --all` exemplar
  • daemon kill/input HTTP rejections → append check \coven daemon status` and `coven sessions` for session state`
  • Ward migrate summary bail → see the report above for per-familiar details
  • run's nothing to do: → nothing to do; (punctuation aligned with the standard)
• Empty states: sessions/status/familiars/skills/memory/research/calls/hub/ward surfaces already print explicit empty lines (verified each renderer in observe.rs and tui/sessions.rs) — left alone. coven pc / pc top / pc disk printed bare section headers on zero rows; they now print one-line No processes to show. / No disks detected. (human mode only).
• Search ingest hint: coven sessions search now prints one stderr note after ingest warnings that fresh TUI transcripts may not be searchable until ingest completes.
• Deliberate skips (kept terse on purpose): --detach/--continue conflict (clap's conflicts_with produces the user-facing error first), non-Unix/Windows platform stubs (kill/attach-input/daemon-serve — factual limits with no next step), ambiguous session-prefix bail (already lists candidate ids), no active session to continue (already has a next step), kill's is not running (status: ...) (already states the blocking status inline), and other modules' errors (out of scope for a reviewable diff).
• Compatibility notes: stderr-only additions in --json-capable paths; stdout JSON bodies byte-identical.

Verification

• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --locked (1269 + suite green)
• python3 scripts/check-secrets.py (passed: no current-tree or history findings)
• Additional manual checks: grepped every changed fragment for pinned test assertions (only prompt must not be empty is pinned, as a prefix, and still passes); verified Cancelled contract block unmodified.

Risk and Rollback

• Risk level: low — message-string and stderr-hint diff.
• Rollback plan: revert the single commit.

Agent Handoff

• Current state: complete; ready for review. Stacked after cli: help text is stale (harness list) and inconsistent across subcommands #448/fix: make CLI help text truthful and consistent #454 (merged).
• Follow-ups: none planned; error copy in other crates/modules could get the same treatment in a future audit.
• Known gaps: hub routing empty state has no next-step line (registering routes isn't a CLI action; kept one factual line).

[Copilot]

Pull request overview

This PR improves the Coven CLI’s human-facing UX by adding “next step” hints to several common error messages and by standardizing empty-state output for coven pc subcommands, without changing control flow, exit codes, or --json stdout bodies.

Changes:

• Add a single stderr “note” after transcript-ingest warnings in coven sessions search, clarifying that very recent TUI transcripts may not be searchable until ingest completes.
• Upgrade multiple anyhow::bail! messages to include semicolon + next-step guidance (examples/commands) while keeping existing behavioral contracts intact.
• Add explicit human-mode empty-state lines for coven pc views that previously printed only section headers when there were zero rows.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/coven-cli/src/main.rs	Adds next-step hints to selected CLI errors and a post-ingest warning note for sessions search (stderr only).
crates/coven-cli/src/pc/display.rs	Standardizes pc/pc top/pc disk human output by printing explicit empty-state lines when there are no processes or disks.

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