fix(stream): don't retry chunk timeouts after text has committed

[yogthos]

Problem

Assistant responses started rendering twice. A response would stream partway, the runtime notices would interleave (compaction / repair / retry banner), then the entire response would re-stream a second time. The first copy is cut off mid-sentence at the stall point — the tell-tale signature.

Root cause

#547 made stream-chunk timeouts retryable even after content had committed (retry.rs):

let retryable = policy.should_retry(attempts, kind)
    && (!committed || is_timeout_error(error));

The premise was that StreamEvent::Retry makes the consumer discard the partial. But the consumer reset (stream.rs PROV-5) only clears the message history — it does not touch text already rendered to the UI. So when attempt 2 re-streamed from scratch, its Token deltas appended right after attempt 1's already-rendered text, producing the duplicate.

Fix

Gate retry on !committed again:

let retryable = policy.should_retry(attempts, kind) && !committed;

The #545 mid-assembly-stall case is preserved: a timeout before any text commits (e.g. a lone tool-call fragment, which is_content_delta does not count as committed) still retries. Only timeouts after committed text now surface instead of duplicating.

Also corrects the now-inaccurate "retried automatically" wording in the timeout error message (rig_stream.rs) and docs/config.md, which only holds for the pre-commit case.

Testing

• Replaced fix(stream): retry mid-assembly chunk timeouts instead of halting #547's retries_timeout_after_content_committed test (which encoded the buggy behavior) with surfaces_timeout_after_content_committed — asserts a post-commit timeout surfaces (counter stays 1, no Retry event, stream ends on Error). Confirmed it fails on the pre-fix code and passes after.
• Added retries_timeout_before_content_committed as a regression guard for the Long Running Tasks: error: stream chunk timed out #545 case — a tool-call-only stream that times out still retries.
• cargo clippy --bin dirge clean, cargo fmt --check clean.
• Full agent::agent_loop::{retry,stream,rig_stream} suites green (48 tests), including the stall-nudge tests.

Closes the regression introduced in #547.