fix(acp): preserve session cost across partial usage updates

[carlos-algms]

• claude streams cost-less usage updates before cost
• blind overwrite wiped cost, header flickered
• shallow-merge usage, keep prior on partial update
• reject cost object without numeric amount

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: a82605c6-0a9b-4fe4-b352-e728d6222703

📥 Commits

Reviewing files that changed from the base of the PR and between 9ca4902 and 2caa676.

📒 Files selected for processing (5)

• .agents/skills/agentic-testing/SKILL.md
• lua/agentic/acp/session_state.lua
• lua/agentic/acp/session_state.test.lua
• lua/agentic/ui/diagnostics_list.test.lua
• tests/integration/test_tool_block_borders.lua

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📝 Walkthrough

Walkthrough

This PR modifies SessionState:set_usage to merge partial usage updates rather than overwrite existing values, adding validation so cost is only applied when cost.amount is numeric, preserving prior cost data otherwise. New tests cover these merge and preservation behaviors. Two test files switch from nvim_win_set_width to nvim_win_set_config for window sizing. The agentic-testing skill documentation is updated to require reconciling expected vs. actual test case counts instead of checking marks via make test.

Possibly related PRs

• carlos-algms/agentic.nvim#139: Overlaps with the diagnostics_list.test.lua truncation window-width test changes.
• carlos-algms/agentic.nvim#262: Related update to the same agentic-testing skill documentation file.
• carlos-algms/agentic.nvim#271: Both touch SessionState in session_state.lua/session_state.test.lua at the same API surface.

Poem

A rabbit hops through usage data, merging fields with care,
No cost gets lost, no size erased — preserved beyond compare.
I nudge the window width with config, tidy and precise,
And count my it()s twice to make the checklist nice.
🐇 EXPECTED meets ACTUAL, matching in the burrow's light!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 75.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: preserving session cost across partial ACP usage updates.
Description check	✅ Passed	The description matches the PR’s core behavior change and test intent, including partial updates and cost validation.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/session-cost-header-flicker

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