feat(subagents): stable color assignment for spawned agents

[emal-avala]

Summary

Slice of Phase 8.10 (subagent color manager piece). Spawned subagents now receive a stable, distinct color across the session — visible in /tasks and propagated to the child via env vars for downstream renderers.

• New services::subagent_colors module: SubagentColor (8 stable slots) and SubagentColorManager (deterministic insertion-order assignment, idempotent per id, wraps after the eighth).
• AppState owns one shared Arc<SubagentColorManager>; ToolContext and TaskContext plumb it through.
• AgentTool resolves a stable subagent id (input field or uuid), assigns a color, propagates AGENT_CODE_SUBAGENT_ID and AGENT_CODE_SUBAGENT_COLOR to the child.
• LocalAgentExecutor pre-assigns a color, registers a LocalAgent queue entry stamped with it (new TaskManager::register_with_color), and drives the entry to a terminal status when the run finishes.
• /tasks rendering colorizes LocalAgent descriptions through the new subagent_theme_color bridge in ui::theme. Shell-task rows stay plain.

/tasks display

After spawning four subagents in a single session the table looks roughly like:

Background tasks (5):

  a1   LocalAgent       3s  running   audit-security      <- red
  a2   LocalAgent      12s  running   refactor-parser     <- blue
  a3   LocalAgent       1s  running   add-tests           <- green
  a4   LocalAgent      45s  completed write-docs          <- yellow
  b5   LocalShell       8s  running   echo plain          <- uncolored

Use TaskOutput to read output; TaskStop to cancel a running task.

Each agent's description is painted with its assigned subagent_* slot from the active theme (the comments on the right are illustrative — there's no extra column). Shell tasks keep the plain rendering they have today.

Out of scope (deferred follow-ups)

• Fork and Resume pieces of 8.10 — not touched here.
• Coordinator memory snapshot piece of 8.10 — not touched here.
• Turn-summary panel colorization in tui::render_turn_summary — needs the subagent id plumbed through ToolEntry / tool-call events. Invasive enough that I shipped the manager + /tasks integration without it; flagging for a follow-up.

Test plan

• cargo check --all-targets
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• cargo test --tests — all new tests green; the only pre-existing failures are bwrap_* sandbox tests that need real user namespaces in the runner (unrelated to this change).
• Unit tests for the manager: 8 distinct ids → 8 distinct colors in declaration order, 9th wraps to red, reassigning an id returns the same color, for_id returns None for unknown ids, concurrent assignment yields 8 distinct slots, Send + Sync compile-time check.
• Unit test for subagent_theme_color: every variant maps to its matching subagent_* slot across every advertised theme.
• Integration test: assign two subagent ids, register both as LocalAgent queue entries with their colors, confirm format_task_list surfaces both rows.