feat: add search commands (task 0016)

Add `search issues <query>` and `search prs <query>`, the full-text escape
hatch the forbidden `--search` flag on the list commands redirects to.

Both hit Gitea's cross-repo issue-search endpoint with the query, a `type`
of issues or pulls, and the owner param, then filter results to the current
repository client-side via each result's `repository` field — the endpoint
has no repo-name filter. The count line reports `count: N of T total` with
`T` from the filtered set (ADR 0005), never the endpoint's cross-repo
`X-Total-Count`.

The positional query is required (VALIDATION_ERROR if missing). Flags:
`--state` (default open), `--label` (comma-separated names passed straight
through as the API `labels` param), `--limit` (default 30), and `--fields`.
Default output is the locator schema (`number`, `title`, `state`, `author`,
`created`) under `issues:` / `pull_requests:` blocks matching the list
commands — search finds the number, `issue view` / `pr view` load the detail.

Covered by fixture-server tests for both types, cross-repo filtering, the
count rule, each flag, the empty state, and missing-query validation, plus
end-to-end tests against a live Gitea instance.

src/cli.ts

@@ -3,6 +3,7 @@ import { exitCodeForError, runAxiCli, AxiError } from "axi-sdk-js";
 import { issueCommand } from "./commands/issue.js";
 import { labelCommand } from "./commands/label.js";
 import { prCommand } from "./commands/pr.js";
+import { searchCommand } from "./commands/search.js";
 import { resolveRepoContext } from "./context.js";
 import type { CliDeps, GlobalFlags } from "./deps.js";
 import { consumeFlagValue, splitFlag } from "./flags.js";
@@ -22,6 +23,8 @@ commands:
   pr comment       Post a comment on a pull request
   label list       List labels in the current repository
   label create     Create a label
+  search issues    Full-text search for issues in the current repository
+  search prs       Full-text search for pull requests in the current repository
 
 global flags:
   -R, --repo <OWNER/NAME>   Override the repository detected from the git origin remote
@@ -119,6 +122,7 @@ export async function runCli(options: RunCliOptions): Promise<number> {
       issue: issueCommand(deps),
       pr: prCommand(deps),
       label: labelCommand(deps),
+      search: searchCommand(deps),
     },
     home: homeCommand(deps),
     stdout: options.stdout,