fix(claude-cli): keep late diagnostics in the CLI exit buffer

[annieyii]

Closes #700. Related: #366, #526.

captureUnparsed filled its 4096 UTF-16 code unit buffer head-first, so with
local hooks active the budget was spent during session startup and whatever
the CLI reported later — including the line naming the failure — was refused.

Three changes to src/lib/claude-cli-transport.ts:

• isDiagnosticLine() classifies the stdout frames the token parser did
  not consume. An explicit error signal (is_error, is_api_error_message,
  outcome: "error", status: "failed", a populated error or stderr
  field, a subtype naming itself) keeps a frame whatever its type. Failing
  that, assistant, user, non-error stream_event and successful result
  frames are dropped by type, as is an allowed rate_limit_event — a
  top-level type whose status is nested under rate_limit_info, and whose
  overageStatus reads "rejected" on a perfectly healthy account. Under
  system only init, hook_started, hook_progress, thinking_tokens
  and a hook_response reporting outcome: "success" count as routine;
  every other subtype is kept, including ones added upstream after this was
  written. That default matters for system specifically: status,
  informational, model_refusal_no_fallback and files_persisted can each
  carry failure information in fields this code has no reason to know about,
  so recognizing errors by enumeration would eat them.

• The buffer evicts from the front instead of refusing to append, so a
  diagnostic that arrives late still fits.

• A single line over budget is truncated to its head AND tail rather than
  its head alone. Hook JSON writes stdout first and its verdict (stderr,
  exit_code, outcome) last, so head-only truncation drops exactly the
  fields that say it failed. The marker counts against the budget, and cuts
  step off surrogate pairs rather than through them.

Not addressed here: why any given CLI invocation fails. This only makes the
existing diagnostic readable. An assistant frame that carries an API error
as text is consumed by the token parser and never reaches this classifier,
which is a separate problem. #526's login failure is unchanged.

Tests

src/lib/__tests__/claude-cli-transport.test.ts, 40 passing. Each of the
three changes was mutation-tested: removing the eviction loop, the head+tail
truncation, or the classifier's default-keep behaviour each turns a dedicated
test red.

Event shapes used as fixtures were taken from live claude CLI 2.1.226
output rather than invented, including the nested rate_limit_info.status
and the thinking_tokens estimated_tokens fields.

Verification

npm run build, npm run test:mocks (132 files / 1893 tests),
npm run mcp:test (22/22), cargo build, cargo fmt --check.