fix(fleet): sanitize slashed branch names in inbox file paths

[Wirasm]

Summary

• Add fleet_safe_name() to replace / with - in branch names used as inbox filenames — refactor/foo becomes refactor-foo.json instead of trying to create refactor/foo.json (nested path that fails with ENOENT)
• Applied consistently across agent flags (--agent-name, --agent-id), inbox creation, config.json entries, inbox removal, and inject writes
• Clarify the dual inbox/dropbox communication flow in the brain agent instructions — previously described as two disconnected systems with no explanation of how they fit together

Test plan

• cargo fmt --check — no violations
• cargo clippy --all -- -D warnings — no warnings
• cargo test --all — all pass (6 new fleet tests)
• Manual: kild create refactor/test-inject --daemon --agent claude creates flat refactor-test-inject.json inbox file
• Manual: kild inject refactor/test-inject "test" delivers to both dropbox and Claude inbox
• Manual: kild inbox refactor/test-inject shows task with correct delivery methods

[Wirasm]

PR Review Summary

Reviewed by 5 specialized agents: code-reviewer, silent-failure-hunter, pr-test-analyzer, comment-analyzer, docs-impact-agent.

Critical Issues: 0

Important Issues: 2

1. DRY violation — fleet_safe_name() duplicated in inject.rs

• crates/kild/src/commands/inject.rs:104 uses branch.replace('/', "-") instead of calling fleet_safe_name()
• fleet is pub(super) in sessions/mod.rs, so inject.rs genuinely can't access it currently
• Fix: re-export fleet_safe_name from kild-core (e.g., pub use fleet::fleet_safe_name; in sessions/mod.rs) and call it in inject.rs
• Without this, the two implementations are coupled by a comment, not the compiler

2. update_team_config missing eprintln! on write/serialize failure

• crates/kild-core/src/sessions/fleet.rs:279-293 — config write and serialize failures log to tracing but produce no user-facing output
• Every other failure path in this file has both warn! + eprintln!; the parallel remove_from_team_config does it correctly
• Pre-existing issue, but this PR touches the function — natural moment to fix

Suggestions

Agent	Suggestion	Location
comment-analyzer	Add fleet_agent_id() to fleet_safe_name() doc comment's usage list	fleet.rs:27-37
comment-analyzer	Update ensure_fleet_member() doc from <branch>.json to <safe_name>.json	fleet.rs:121-127
comment-analyzer	Update remove_fleet_member() doc similarly	fleet.rs:319-321
comment-analyzer	Update fleet_agent_id() doc from <branch>@honryu to <safe_name>@honryu	fleet.rs:297-300
docs-impact	Update CLAUDE.md lines 418+420 — fleet section still says <branch> for agent flags and inbox path	CLAUDE.md:418-420

Test Coverage: Adequate, 2 improvements suggested

Rating	Suggestion
7/10	Add remove_fleet_member round-trip test with slashed branch (create → remove → assert gone)
6/10	Add inject.rs test for slashed branch sanitization as divergence canary

Strengths

• Core fix is correct and consistently applied across all 5 callsites in fleet.rs
• Tests are well-targeted: positive + negative assertions, identity + edge cases
• kild-brain.md rewrite clearly explains the dual inbox/dropbox flow
• Error handling in new code paths is solid (no silent failures introduced)

Verdict: NEEDS FIXES — address the 2 important issues, then good to merge