feat(memory): add per-session disable policy

[BrendanChou]

Summary

• add a sticky per-session memory policy with configured and disabled states
• add --no-memory / TACT_NO_MEMORY plus a discoverable alternate New session action
• preserve the restriction across resume, forks, handoffs, model changes, and child agents
• keep the global memory browser and ordinary session persistence independent

Disabled sessions do not construct a memory backend, expose the memory tool, include memory instructions, or receive memory-review checkpoints. This is intentionally not an incognito mode: transcripts, checkpoints, history tools, provider requests, and filesystem effects remain.

Testing

• just check-fmt
• cargo check --all-features
• cargo test -p tact (773 unit tests, 15 release-pipeline tests)

Stack

Depends on #155. Retarget to main after #155 merges.