fix(agent): balance history when a tool-call wave aborts mid-flight

[hung12ct]

What & why

When the anti-loop detector aborts a turn mid-wave, the loop persisted the assistant message with its tool_calls and no tool results at all, and discarded every result that had already completed in that wave.

Two consequences. The saved transcript is unbalanced, which providers reject — the only reason it does not surface today is that pkg/llm/anthropic repairs history on every call. And completed tool work (a query that ran, a subprocess that finished) is silently thrown away, so the next turn's model never sees results the user already paid for.

Changes

• synthesizeMissingToolErrors fills any call with no recorded result, leaving completed results untouched. Generalises the existing synthesizeDroppedToolErrors pattern, which covered only the per-turn budget-drop path.
• The fatal path in runIterationOnce now drains ws.toolMsgs through appendToolResultsInOrder before saveSession, so history leaves the loop balanced by construction.
• Skipped calls carry a reason naming the abort, so a resumed session does not read the gap as a silent success.

Deliberately not in this PR: removing the defensive PatchDanglingToolCalls call in pkg/llm/anthropic. It is now redundant for this path, but that is a critical-package edit and the two other callers (regenerate.go, loop_stream.go) have not been proven redundant.

Testing

• TestFatalWave_SavesBalancedHistory — drives a 6-call wave of identical calls so the detector trips partway through, then asserts every tool_call in the saved history has a matching tool result, that at least one real result survived, and that the total is exactly 6. Verified to fail on the pre-fix code: tool_call "dup0" has no matching tool result in saved history (3 messages).
• TestSynthesizeMissingToolErrors_PreservesCompleted / _FillsEmptyMap — gap-filling never overwrites a recorded result; an empty map still yields a balanced transcript.
• gofmt -l . empty, make lint 0 issues, make build, make test, make test-race all clean.

Checklist

• Title is a Conventional Commit
• gofmt -l . prints nothing
• make lint is clean
• make build passes
• make test (and make test-race) passes
• Errors wrapped with a package prefix
• CHANGELOG.md updated if this PR is cut as a release tag
• No changes to pkg/llm/, pkg/history/, or pkg/telemetry/