fix: harden /handoff command against DCP context compression

[jflowers]

Summary

Hardens the /handoff command prompt against DCP context compression to prevent step reordering, template loss, and unsafe reservation releases during compressed sessions.

Under DCP context compression, the original handoff.md suffered three fragilities: numbered steps could be reordered or merged, the handoff note template section could be dropped entirely, and no precondition check prevented releasing reservations while forge workers were active. This is the same class of vulnerability as unbound-force/unbound-force#346 (review-pr confirmation gate bypass).

Fixes #50.

How to Test

# Run the structural content test
go test ./internal/agentkit/ -run TestHandoffMD -v

# Run the full test suite
make test

# Verify the hardened file structure
cat internal/agentkit/content/commands/handoff.md

Verify the following structural properties:

1. "Steps MUST execute in this exact order" appears before the first numbered step
2. All five handoff note categories (Completed, In Progress, Blocked, Next Steps, Gotchas) appear within the org_session_end step section
3. No separate "## Handoff Note Template" section exists
4. "Forge precondition check" text is present before step 1
5. "Depends on step N" rationale exists for steps 2-5

How to Demo

Read the hardened handoff.md file and observe the structural changes: explicit ordering constraint, forge precondition check (step 0), per-step dependency rationale, and inlined handoff note template in step 5. Run the test to see the 5 structural assertions pass.

Key Files Changed

File	Change
internal/agentkit/content/commands/handoff.md	Hardened prompt: ordering constraint, forge precondition, step rationale, inline template
internal/agentkit/agentkit_test.go	New TestHandoffMD_StructuralHardening with 5 assertions
openspec/changes/harden-handoff-dcp/proposal.md	Change proposal with constitution alignment
openspec/changes/harden-handoff-dcp/design.md	Technical design (4 decisions, 3 risks)
openspec/changes/harden-handoff-dcp/specs/*.md	Delta spec with 4 requirements and scenarios
openspec/changes/harden-handoff-dcp/tasks.md	Implementation task checklist (all complete)

This PR was generated by /uf.finale (AI-assisted).