Write tools report a failed write as a success when fallback is set, and return gh's stderr in successful results #22

25 of the 26 tools in tools-write.json take a fallback parameter. On a write tool it turns a failed write into a successful result that carries the caller's text, so an agent cannot tell a failed write from one that went through. pr_merge with fallback: "merged" on a PR that cannot be merged returns isError: false with the text merged.

The same tools also return gh's stderr inside a successful result. When --head is not passed and the working tree has uncommitted changes, gh pr create prints Warning: N uncommitted changes on stderr (pkg/cmd/pr/create/create.go in cli/cli), and pr_create returns that line together with the PR URL.

Cause

• fallback: each write tool copies the same execution block, whose failure branch prints fallback and returns 0: [[ -n "${fallback}" ]] && { echo "${fallback}"; return 0; }, or the same with printf '%s\n'. Examples: lib/pr_write.sh:73, lib/issue_write.sh:63, lib/review_write.sh:129, lib/label.sh:115 (shared by the label and assignee tools), lib/issue_schema_write.sh:295, lib/sub_issue_write.sh:95, lib/api.sh:88, and lib/project.sh:238, :311 and :331. Because the tool exits 0, shared/mcpserver_core.sh:939 answers "isError": false. Without fallback, a non-zero exit becomes "isError": true (mcpserver_core.sh:932-933).
• stderr: PR fix(github-mcp): stop hiding gh failures and validate tool input earlier #13 moved the values it lists (sub-issue node IDs, the head SHA pr_review_submit fetches, the project and status listings, among others) onto _gh_capture_split. The success path of the copied block was left as it was: 22 copies in the write tools run __raw=$("${cmd[@]}" 2>&1), and 20 of them return __raw unchanged (for example lib/pr_write.sh:70). Two write paths parse the merged stream: _gh_issue_schema_write runs jq over it and returns the raw text when jq fails (lib/issue_schema_write.sh:302), and project_status_set pipes gh project item-list ... 2>&1 into jq (lib/project.sh:307).

comment_edit (PR #21) already does what this issue asks for: it declares neither fallback nor suppress_errors, and it reads stdout and stderr apart (lib/review_write.sh:256).

Fix

• Remove fallback from the schema of the 25 write tools in tools-write.json, and remove the fallback branch from each tool_*() function and from _gh_edit_list_param and _gh_issue_schema_write. Cost: every write tool sets additionalProperties: false, so a call that still passes fallback fails with Unknown parameter(s): fallback. REFERENCE.md:583, which describes fallback for write tools, changes with it.
• In every write tool, capture gh's output with _gh_capture_split. On success, return stdout. On failure, return stderr, or stdout when stderr is empty, because gh api prints an HTTP error's JSON body on stdout (lib/issue_schema_write.sh:296-297). Apply the same split to the item-list lookup at lib/project.sh:307.
• Tests: per tool, a call that passes fallback is rejected by schema validation, and a gh stub that writes a warning to stderr and exits 0 returns only stdout.