bug: selecting auto reasoning effort inherits the previous override

Problem

Selecting auto in the mecatui /effort picker does not clear an explicit reasoning-effort override.

The UI encodes auto as an empty string, but the fork API and server interpret an empty reasoning_effort as “inherit the source session's effort.” The operation still creates and adopts a successor session, while its effective effort remains unchanged.

Reproduction

1. Start a session with an explicit reasoning effort, for example high.
2. Open /effort.
3. Select auto.
4. Inspect the effective effort on the successor session.

Actual behavior

A successor session is created, but it inherits high.

Expected behavior

An explicit auto selection clears the session override and lets the operator/provider default apply.

Cause

• The picker maps auto to "": cmd/mecatui/ui/effort.go (effortValue) and cmd/mecatui/ui/effort_test.go (TestEffortPickAutoSendsEmpty).
• successorProviderSelector only overrides the source effort when the request value is non-empty: internal/adapter/server/placement_successor.go.
• ForkSessionRequest.reasoning_effort is a non-presence-aware string, so omitted/inherit cannot be distinguished from explicitly empty/reset: contracts/proto/mecatl/v1/harness.proto.

Acceptance criteria

• The fork contract distinguishes “inherit source effort” from “explicitly reset to auto.”
• Selecting auto after an explicit effort produces a successor whose effective effort reflects the operator/provider default rather than the source override.
• Omitting an effort override continues to inherit the source effort.
• gRPC and HTTP behavior agree.
• Tests cover explicit reset, omitted inheritance, and a non-empty override.

Start with cmd/mecatui/ui/effort.go and effort_test.go, then trace ForkSessionRequest in contracts/proto/mecatl/v1/harness.proto into internal/adapter/server/placement_successor.go. Run the relevant Go tests and inspect the gRPC and HTTP paths. Done means explicit auto resets the effort, omission inherits it, non-empty overrides still work, and tests cover all three cases.