refactor: extract magic strings, decompose long functions, standardize error display

[Wirasm]

Summary

• Extract magic strings into constants: WORKTREE_ADMIN_PREFIX, SHIM_VERSION, and use kild_branch_name() / KILD_BRANCH_PREFIX consistently instead of raw format!("kild/...") literals
• Decompose 3 long functions: kill_tracked_agents() and sweep_ui_daemon_sessions() from destroy_session(), resolve_resume_args() from open_session()
• Standardize CLI error display with display_operation_error() helper and consistent color::error() usage across 9 command handlers

Test plan

• cargo fmt --check passes
• cargo clippy --all -- -D warnings passes
• cargo test --all passes (3 pre-existing env-dependent test failures unrelated to this change)
• No behavioral changes — pure refactor

Closes #438

[Wirasm]

PR Review Summary

Reviewed by: code-reviewer, docs-impact-agent, comment-analyzer, code-simplifier

Critical Issues (0 found)

None. Code review confirms the refactoring is behavior-preserving across all 18 changed files.

Important Issues (3 found)

Agent	Issue	Location
comment-analyzer	kill_tracked_agents docstring implies force controls daemon error suppression, but daemon errors are always non-fatal regardless of force	crates/kild-core/src/sessions/destroy.rs:37-40
comment-analyzer	resolve_resume_args docstring omits is_bare_shell parameter entirely and hides both error return paths (ResumeUnsupported, ResumeNoSessionId) — describes 2 branches when there are 3	crates/kild-core/src/sessions/open.rs:39-46
docs-impact	kild-git crate is absent from CLAUDE.md workspace structure list despite being a real workspace member used by kild-core, kild-ui, and kild-tmux-shim. PR #628 adds WORKTREE_ADMIN_PREFIX there, making the gap more conspicuous	CLAUDE.md workspace structure section

Suggestions (5 found)

Agent	Suggestion	Location
simplifier	.unwrap() in kill_tracked_agents on kill_errors.first() — logically safe but violates the "no .unwrap() in production" rule. Use &kill_errors[0] or let-else	crates/kild-core/src/sessions/destroy.rs:142
simplifier	display_operation_error takes &dyn std::fmt::Display — prefer impl std::fmt::Display for monomorphization (all callers pass concrete types)	crates/kild/src/commands/helpers.rs:19
simplifier	WORKTREE_ADMIN_PREFIX is pub(crate) with a single use site inside kild_worktree_admin_name() — adds indirection without external callers to justify it. Consider inlining	crates/kild-git/src/naming.rs:18
simplifier	pr.rs calls kild_branch_name(branch) in no_pr_found path when kild_branch variable is already in scope — reuse the variable	crates/kild/src/commands/pr.rs
simplifier	teammates.rs uses crate::color::error when color is already imported — use color::error	crates/kild/src/commands/teammates.rs:51

Strengths

• Function decomposition is clean — kill_tracked_agents, sweep_ui_daemon_sessions, resolve_resume_args have good names and well-scoped responsibilities
• display_operation_error helper standardizes CLI error output with a clear format contract
• Magic string extraction to kild_branch_name()/kild_worktree_admin_name() eliminates scattered format!("kild/...") calls
• sweep_ui_daemon_sessions docstring precisely documents the {kild_id}_ui_shell_{counter} naming convention — excellent load-bearing context

Documentation Issues

• CLAUDE.md workspace structure: add kild-git entry
• CLAUDE.md git/ module description: note that naming utilities now live in separate kild-git crate

Verdict

READY TO MERGE — no behavioral issues. The important issues are docstring accuracy improvements and a CLAUDE.md gap that should be addressed (in this PR or a follow-up).