mcp: embed example calls in top-level tool descriptions (#723)

* mcp: embed example calls in top-level tool descriptions

agents (esp. claude sonnet) hallucinate param names from training-data
priors — `pr_number` instead of `pull_number`, `summary` instead of
`body`, full subcommand strings jammed into `git({command})` like it
were `shell({command})`. each error burns a tool round-trip plus a
follow-up ToolSearch, ~40+ events / 24h, no observable recovery cost
to us but visible to users in agent logs.

cheapest fix: add a sample formatted function call to every affected
tool's top-level description. example anchors are more reliable than
schema descriptions alone because the model treats descriptions as
narrative but call examples as canonical structure. for `git` and
`shell` (whose `command` fields collide), include explicit
counter-examples disambiguating which tool owns which shape.

no schema aliases / coercion yet — try the cheap thing first; if the
next audit window still shows the same hallucination rate, layer
aliases on top per #585's recommendation.

closes #585, closes #701

* mcp: drop negative anchors from tool descriptions

negation is a footgun in tool descriptions — telling the model "NOT
pr_number" makes pr_number more salient, not less. let the positive
example carry the schema and trust the model to read it.

removes:
- "the parameter is pull_number (a number), NOT pr_number" and
  similar across checkout_pr, get_pull_request, list_pull_request_reviews,
  get_review_comments, create_pull_request_review
- "NOT summary, message, or content" on report_progress
- "WRONG: git({ command: 'log --oneline' })" counter-example on git
- redundant param-type restatements after the example (e.g. "depth is a
  number, not a string" on git_fetch, "description is required" on shell)

keeps a single positive example per tool. for tools with multiple call
shapes (git, git_fetch, push_branch), two positive examples instead of
one + a counter-example.

mcp/shell.ts

@@ -189,6 +189,8 @@ export function ShellTool(ctx: ToolContext) {
     name: "shell",
     description: `Execute shell commands securely. Environment is filtered to remove API keys and secrets.
 
+Example: \`shell({ command: "pnpm test", description: "run the test suite" })\`.
+
 Use this tool to:
 - Run shell commands (ls, cat, grep, find, etc.)
 - Execute build tools (npm, pnpm, cargo, make, etc.)