Add Cline harness over its native ACP server (cline --acp)

[jhonoryza]

Closes #236

Summary

Adds Cline (open-source coding agent, npm cline, Apache-2.0) as a harness, driven over its native ACP server — cline --acp — via a new AcpAgentSpec on the shared AcpHarness (same shape as Grok/Hermes/Pi). No new driver; no engine/sync/doc-crate changes.

• HarnessId::Cline (proto, serde kebab-case → "cline", additive + wire-compatible)
• cline_spec(): executable cline, args ["--acp"], env override CLINE_EXECUTABLE, TurnBoundary steering, reasoning ladder Minimal..XHigh mapped from the CLI's --thinking (degrades to agent default — no config option advertised), fallback "Cline default" model, 30s prompt stall bound
• Engine registry slot + agent-account slug + UI surfaces (settings blurb, cli_name, brand icon cline-mark, picker + accounts icons) + ZERON_HARNESS=cline
• Fixture test cline_spec_runs_the_same_fixture_turn (existing fake-ACP harness)
• Docs: feasibility analysis + task breakdown + live-probe results (docs/research/cline-harness.md, docs/research/cline-harness-tasks.md)

Verified against the real CLI (3.0.60, live probe — details in the doc)

• session/prompt settles deterministically with stopReason: "end_turn" (4.1s trivial turn)
• First-class models catalog + plan/act modes on session/new; zeron's picker gets the real catalog
• configOptions are provider + model only — approvals always arrive as session/request_permission and bridge to zeron's approval UI (deliberately NOT launching with --auto-approve true)
• Cross-process session/load resume confirmed (replays prior conversation; mcpServers required)
• No steering extension (_meta null) → turn-boundary steering, as spec'd
• promptCapabilities.image: true → attachments map to image content blocks

Testing

• cargo check --workspace clean
• cargo test -p zeron-harness green (incl. new cline fixture test)
• cargo test -p zeron-engine registry green
• cargo clippy — no new warnings from this change (remaining warnings are pre-existing in untouched files)
• Live end-to-end against the real cline --acp: handshake → session/new → session/prompt → cross-process session/load + follow-up turn

Known follow-up

• cline installed via mise is only reachable through the login-shell PATH probe; consider adding mise bin dirs to the shared version-manager scan

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}