refactor: extract shared kild-protocol crate for IPC message types

[Wirasm]

Summary

• Extract ClientMessage, DaemonMessage, and SessionInfo wire types from kild-daemon into a new lightweight kild-protocol crate (serde + serde_json only)
• Replace manual json!() construction and .get() chains in kild-core and kild-tmux-shim with typed enum construction and pattern matching
• Protocol changes are now compile-checked across all three consumers — no more silent drift

Changes

Crate	What changed
kild-protocol (new)	ClientMessage, DaemonMessage, SessionInfo + 18 serde roundtrip tests
kild-daemon	Re-exports types from kild-protocol, removed moved type definitions
kild-core	send_request() accepts &ClientMessage, returns DaemonMessage
kild-tmux-shim	Same typed rewrite for all IPC functions

Test plan

• cargo fmt --check — clean
• cargo clippy --all -- -D warnings — 0 warnings
• cargo test --all — 1655+ tests pass, 0 failures
• cargo build --all — full workspace builds
• Wire format unchanged (existing roundtrip tests prove this)

Closes #328

[Wirasm]

PR Review Summary

Critical Issues (2 found)

Agent	Issue	Location
silent-failure-hunter	read_scrollback: base64 decode failure silently returns empty data via .unwrap_or_default()	crates/kild-core/src/daemon/client.rs:421-423
silent-failure-hunter	read_scrollback: unexpected response type silently returns Ok(Some(Vec::new())) instead of error	crates/kild-core/src/daemon/client.rs:426

Important Issues (2 found)

Agent	Issue	Location
silent-failure-hunter	get_session_info: unexpected response type silently returns Ok(None) (looks like "not found")	crates/kild-core/src/daemon/client.rs:388
silent-failure-hunter	get_session_status: unexpected response logged at debug level but returns Ok(None) with misleading "completed" event name	crates/kild-core/src/daemon/client.rs:333-339

Suggestions (2 found)

Agent	Suggestion	Location
type-design-analyzer	SessionInfo.status should be a typed enum (e.g. SessionStatus::Running { pty_pid } / SessionStatus::Stopped { exit_code }) to make illegal states unrepresentable	crates/kild-protocol/src/types.rs:13
type-design-analyzer	DaemonMessage::Error.code should be a typed enum instead of String to eliminate brittle string matching in client code	crates/kild-protocol/src/messages.rs:153-158

Strengths

• Clean extraction of wire types into zero-dependency protocol crate (serde only)
• Correct dependency graph — no circular deps, kild-daemon re-exports from kild-protocol
• Comprehensive serde roundtrip tests (18 tests) covering all message variants + edge cases
• Typed enum construction eliminates entire classes of runtime bugs (typos, missing fields, type coercion)
• CLAUDE.md updates are accurate and complete
• Existing 14 integration tests provide behavioral regression safety

Documentation Updates

• CLAUDE.md — Already updated with kild-protocol crate description, build commands, and architecture notes. All changes verified accurate.

Agent Results

Agent	Verdict
code-reviewer	PASS — no high-confidence issues
docs-impact-agent	No updates needed — documentation accurate
silent-failure-hunter	2 CRITICAL + 2 HIGH — silent failures in client.rs
pr-test-analyzer	GOOD COVERAGE — no additional tests required
type-design-analyzer	B+ (7/10) — adequate, SessionInfo status typing recommended

Verdict

NEEDS FIXES — 2 critical silent failures in read_scrollback must be addressed before merge. The base64 .unwrap_or_default() and catch-all Ok(Some(Vec::new())) both hide real errors. Note: the shim's read_scrollback already handles base64 decode correctly — match that pattern.

Recommended Actions

1. Fix read_scrollback base64 decode: replace .unwrap_or_default() with .map_err() (match shim pattern)
2. Fix read_scrollback catch-all: return ProtocolError for unexpected response types instead of empty vec
3. Fix get_session_info and get_session_status catch-all arms to return errors (or at minimum warn! with _failed event name)
4. Consider typed SessionStatus enum for SessionInfo in a follow-up