[agent] Truncate tool output to a token budget, keeping head, tail and a summary #304

Context

Roadmap B6 (anchor #168): tool output is truncated by a line cap and a byte cap, not by what the model
pays for, and a truncated result is a head-only slice of raw text.

agent/truncate.go splits on lines and keeps the first max_lines / max_bytes; the rest is written to
a file and the model is told where it went. Two consequences:

• The preview's size is measured in bytes the model does not pay for directly: 200 KiB of dense text is
  ~50k tokens, which is a whole context window spent on one tool result.
• The kept part is the head. A failing command's evidence — the panic, the FAIL, the last warning
  — is at the end, and a head-only preview drops exactly the lines a reader needs.

What to build

• A token budget beside the line and byte caps (truncation.max_tokens, default 4000, measured with
  the same estimator the compression threshold uses, EstimateTokens). The budget covers the whole
  preview: the tail is taken first, the head gets what is left.
• A tail (truncation.tail_lines, default 20): the last lines of the output, so a failure survives
  truncation.
• A summary of what was omitted, not just a byte count: how many lines were dropped, and how many of
  them mention an error or a warning. That is a deterministic scan, not a model call — the preview must
  stay cheap and reproducible.
• The saved file, its 0600 mode and the [TRUNCATED] pointer stay exactly as they are.

Acceptance

• An output past max_tokens produces a preview whose EstimateTokens is within the budget (the tail
  included), and the last line of the output is present in it.
• The omitted-lines line reports the real count and the error/warning counts of the dropped lines.
• An output inside every bound is returned unchanged, byte for byte, with no file written.
• A single line larger than the byte cap still yields a preview that starts with that line's first
  characters (never an empty preview).
• SetTruncationConfig keeps its rule that zero or negative values do not lower a bound.
• The config defaults and the package defaults are asserted equal.
• go test ./... -count=1, -race, go vet, gofmt, staticcheck clean.

Out of scope

Summarising with a model (the preview must be free and deterministic), and per-tool budgets: this is one
budget for every tool result.