[APP-5893] Reduce Merkle snapshot deserialization memory

[warp-agent-staging]

Description

Merkle snapshot reconstruction now reserves each serialized node's exact child capacity and shares one Arc<Path> across a file node, its fragment nodes, and reconstructed fragment metadata. This removes repeated path allocations without changing the serialized snapshot schema or path-based behavior.

The separate serde-buffered AIAgentContext / PersistedAIInputType / SkillDescriptor allocation cluster remains follow-up work and is intentionally out of scope.

Linked Issue

APP-5893

• The linked issue is labeled ready-to-spec or ready-to-implement. — Linear issue; the GitHub label does not apply.
• Where appropriate, screenshots or a short video of the implementation are included below. — Not applicable; no UI changes.

Testing

• ./script/format — passed

• cargo nextest run --manifest-path /workspace/warp/Cargo.toml -p ai — 385 passed, 0 skipped

• cargo clippy --manifest-path /workspace/warp/Cargo.toml -p ai --all-targets --features local_fs -- -D warnings — passed

• cargo clippy --manifest-path /workspace/warp/Cargo.toml -p warp --all-targets -- -D warnings — passed

• Full workspace all-features Clippy was attempted but is blocked by existing agent_mode_evals-gated execution-profile test imports and unrelated dead-code errors.

• I have manually tested my changes locally with ./script/run — Not applicable to snapshot allocation behavior.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

CHANGELOG-NONE

[warp-agent-staging]

This PR was generated with Warp.

Comment @warp-staging-factory on this PR to send it follow-up work.