fix(core): improve ask caller routing and cross-dir session resolution

[daniellee2015]

Summary

This PR hardens async task routing and session resolution in core flows.

Problem

1. In nested-directory and cross-pane scenarios, ask can route to the wrong caller/session
   when CCB_CALLER is missing or incomplete.
2. Users may see processing... while the target session never actually starts work.
3. On single-instance lock collisions, current behavior hard-fails instead of reusing the
   existing provider pane.

Files changed

1. bin/ask
2. bin/ccb-completion-hook
3. ccb
4. lib/session_utils.py

What changed

1. bin/ask

2. Added robust caller detection (CCB_CALLER + pane/env hints + fallback).

3. Removed strict hard-fail when caller is missing; use controlled fallback caller.

4. Added async observability output (task status/log file paths and lifecycle markers).

5. Guardrail text is now strict-by-default only for Claude caller to reduce false guidance for
   other callers.

6. bin/ccb-completion-hook

7. Improved session file lookup and work-dir compatibility checks for nested-dir workflows.

8. lib/session_utils.py

9. find_project_session_file now searches upward from cwd and picks nearest valid session
   file.

10. ccb

11. On lock collision in single-provider mode, try to reuse/activate existing live pane
    instead of hard-failing.

12. Explicitly inject CCB_CALLER into provider pane env to reduce caller ambiguity across
    processes.

13. OpenCode startup path is hardened in single-provider handoff scenarios.

User impact

1. Better routing from subdirectories.
2. Faster diagnosis when tasks appear stuck (status/log lifecycle is visible).
3. Fewer hard failures when an existing ccb instance is already active.

Risk

1. Low-to-medium; mostly default routing/session resolution behavior.
2. Changes are additive with fallbacks.
3. No protocol-level model behavior changes.

Out of scope

1. No ccb-status submodule updates.
2. No ccb-multi, worktree, or context-related changes.
3. No non-core package changes.

Validation

1. Python syntax checks passed for all touched files.
2. Verified nested-dir session routing behavior.
3. Verified single-provider lock-collision pane reuse path.