Add immutable UUIDs + monotonic event IDs for branch-safe agent workflows

[vikasprogrammer]

Problem

Task IDs are currently sequential integers from a counter in config.yml. This creates two issues for multi-agent/multi-branch workflows:

Branch ID collisions

Two agents on separate branches both create TSK-3 → merge conflict, potential data loss. The counter in config.yml and the task filenames (3.yml) will collide.

No retry idempotency

If an agent's tsk move call times out and it retries, there's no way to detect the duplicate. The activity log gets a double entry.

Proposed solution

Immutable UUIDs as internal keys

• Each task gets a UUID at creation time (stored in the YAML)
• TSK-N remains the friendly display label (no UX change)
• File storage keyed by UUID instead of integer ID
• Merges become conflict-free — two branches can't create the same UUID

Monotonic event IDs on activity entries

• Each activity log entry gets a unique, monotonically increasing event ID
• Enables deduplication on retry ("already applied")
• Makes it possible to detect missing events after merges

Design considerations

• Backward compatibility: existing TSK-N references should keep working
• The config.yml counter can remain for generating friendly labels, but shouldn't be the source of truth for identity
• UUIDs could be short (nanoid-style) to keep filenames readable

Context

This becomes important as tsk positions toward multi-agent workflows where agents work on parallel branches and merge results. Single-branch usage (most current users) is unaffected.