ttl: persist dependency edges as dep task keys in task_deps

[kdy1]

Summary

task_deps.dep_task_key is currently populated from task dependency IDs, not canonical dependency task keys.

Evidence

Contract Evidence:

• docs/project-ttl-language.md:182
• cmds/ttlc/internal/cache/sqlite.go:377

Current Gap

task_deps.dep_task_key is currently populated from task dependency IDs, not canonical dependency task keys.

Proposed Scope

• Not documented yet.

Acceptance Criteria

• Resolve dependency IDs to canonical dependency task keys before persisting task_deps.
• Preserve consistency between task_cache.task_key and task_deps.dep_task_key values.
• Handle missing dependency key mapping with deterministic diagnostics/failures.
• Provide migration or compatibility handling for existing rows as needed.

Test Scenarios

• Inserted dependencies reference actual task keys in sqlite rows.
• Rebuild updates dependency rows when task key changes.
• Missing dependency mapping triggers deterministic failure path.

Out of Scope

• Schema redesign unrelated to dependency key mapping.
• Non-SQLite backend support.