mcp: restore operational guidance dropped in #723

#723's revision pass cut four substantive strings along with the
negative anchors. those strings address real, audit-observed failure
modes and the positive examples don't carry them.

restored:
- push_branch: "if the response reports a timeout, the underlying
  push may have actually succeeded — verify with git log
  origin/<branch> before retrying" (was on the tool description)
- create_pull_request_review commit_id .describe(): "must be the FULL
  40-character SHA — abbreviated SHAs are rejected by GitHub with 422"
- create_pull_request_review comments[].line .describe(): "must sit
  inside a `@@` hunk... dropped entries are reported under
  droppedComments in the response"
- create_pull_request_review comments[].start_line .describe(): "both
  start_line and line must sit inside the same @@ hunk"

also: get_commit_info example used a 31-character SHA (non-standard
truncation). swapped to a 7-char short form, which is what git
log --oneline emits and what agents see in practice. note that this
tool accepts either full or abbreviated, unlike create_pull_request_review
which requires full.

mcp/commitInfo.ts

@@ -16,7 +16,7 @@ export function CommitInfoTool(ctx: ToolContext) {
     description:
       "Retrieve commit metadata and diff via GitHub API. Use this instead of git show for reviewing commits - " +
       "it works with shallow clones and shows the actual changes in the commit. Returns diffPath pointing to formatted diff file. " +
-      'Example: `get_commit_info({ sha: "2a6ab5d06ba64dd987bd88c4403287" })`.',
+      'Example: `get_commit_info({ sha: "2a6ab5d" })`.',
     parameters: CommitInfo,
     execute: execute(async ({ sha }) => {
       const response = await ctx.octokit.rest.repos.getCommit({