CL-8980: make read_file continuation failures recoverable rather than terminal

[TheGreatAxios]

Truncated reads minted opaque one-shot in-memory handles: after session resume, prune, or compaction the URI was a bare missing blob with no source or offset named, and the notice hid the original path, leaving no fallback.

What changed:

• Handles are self-describing (tool-output:///cursor/ carrying source, offset, window limit, nonce), so a verbatim follow serves the next window with no in-memory record — across resume, replay, and compaction.
• Dead/unknown handles fail as isError naming the source and resume offset, never a bare missing blob. URIs that were never handles still fail as missing blobs.
• Spent-handle replay stays a single-use error with one followable next call.
• Truncation notices carry a plain path+offset fallback alongside the handle.
• Compaction stubs preserve resume recipes (handles/spill URIs) instead of hollowing them.

Verification:

• RED commit be9dca1 pinned the contract first (targeted run failed before the fix).
• bun run typecheck: exit 0 (also via pre-commit).
• Targeted (vendor/intx-tools-posix tools-posix, lazy-blob-reader, codex-tool-proxies, all of src/plugins/): 644 pass, 0 fail.
• Full: bun test ./src ./tests ./evals ./scripts --randomize --seed 424242: 8043 pass, 0 fail.
• bun run lint (oxfmt --check + oxlint): 0 warnings, 0 errors.
• read_file continuation isErrors carry none of the four decline markers classifyDeclinedToolResult matches on, so director.ts stays out of this path (verified by marker-exclusion; the classifier is module-private).
• Past-EOF contract shared with the CL-8979 lane ([offset N is beyond end of file (M lines)], isError) is untouched; resumed handles serve through the same readFileBounded path.

Sibling note: CL-8979 works the same read_file area on a separate branch; this diff is recovery behavior only.

Fixes CL-8980

[linear-code]

CL-8980

[TheGreatAxios]

Superseded by #1179 (single-way path+offset resume). Closing per release review.