[10/10] --exclude-team-prefixes: exclude noisy team-name prefixes from grouping #200

Context

Real-world team structures can have many closely related, deeply-overlapping team names under one prefix (e.g. chapter-validators-core, chapter-validators-client, chapter-validators-frontend-client, chapter-validators, plus unrelated chapter-secops, chapter-head-of-frontend, ...). When several of these co-occur on the same repos, --group-by-team-prefix produces many distinct, hard-to-read combined sections ("a + b + c"), and --pick-team-auto (#197) cannot resolve most of them because no single candidate is a literal string-prefix of every other candidate in the combo (see the investigation test added in #197: chapter-secops + chapter-validators-core, chapter-head-of-frontend + chapter-secops + chapter-validators-core, chapter-secops + chapter-validators + chapter-validators-core all stay unresolved).

Rather than trying to build a more aggressive auto-merge heuristic (which risks silently discarding real team-membership information — e.g. collapsing 3 distinct combos that share only some members into one arbitrary label), the more principled fix is to let the user exclude noisy team-name prefixes from grouping entirely before combos are even formed. This directly reduces the number of ambiguous combined sections at the source, and makes the remaining ones easier for --pick-team / --pick-team-auto to handle.

Part of EPIC #125.

Solution

Add a new repeatable/comma-separated CLI option, --exclude-team-prefixes <prefixes> (plural, for naming consistency with --exclude-repositories / --exclude-extracts — note the original ask used the singular --exclude-team-prefix; confirm the final name before implementing), that filters out any team matching one of the given prefixes before groupByTeamHierarchy runs:

• A new pure function in src/group.ts, e.g. excludeTeamsByPrefix(groups: RepoGroup[], excludePrefixes: string[]): RepoGroup[], returning new RepoGroup objects with teams filtered to drop any team whose name starts with one of excludePrefixes (same case-sensitive startsWith semantics as bucketSingleLevel's prefix matching, for consistency with --group-by-team-prefix).
• Wire it in github-code-search.ts: parse the option (comma-separated, trimmed, same pattern as --exclude-repositories), and apply excludeTeamsByPrefix to groups right after g.teams = teamMap.get(...) is assigned and before groupByTeamHierarchy(groups, chains) is called.
• A repo whose entire team list is excluded falls through to "other", same as a repo with no matching team today — no special-casing needed beyond the filter itself.
• Only applies with --group-by-team-prefix (mirrors --pick-team / --pick-team-auto's existing "requires --group-by-team-prefix" guard and warning).
• Update the replay command (ReplayOptions in src/output.ts) so a session using this flag reproduces identically.
• Update src/completions.ts, docs/usage/team-grouping.md, and docs/reference/cli-options.md.

Acceptance Criteria

• --exclude-team-prefixes "chapter-validators-" removes every team starting with that prefix from consideration for grouping, for every repo.
• A repo left with zero matching teams after exclusion falls into "other", exactly like a repo with no matching team today.
• Excluding a prefix reduces (or eliminates) combined sections that were only ambiguous because of the excluded teams, without affecting sections unrelated to the excluded prefixes.
• Multiple exclude prefixes are supported (comma-separated).
• A warning is emitted (and the flag is a no-op) when used without --group-by-team-prefix, mirroring --pick-team.
• The replay command reproduces a session using this flag exactly.

Definition of Done

• Unit tests for excludeTeamsByPrefix in src/group.test.ts: removes matching teams, keeps non-matching teams, drops a repo's entire team list correctly (falls to "other" after grouping), no-op with an empty exclude list, multiple prefixes, pure/no-mutation.
• CLI wiring + replay-command test coverage (src/output.test.ts, src/completions.test.ts).
• docs/usage/team-grouping.md and docs/reference/cli-options.md updated.
• bun test, bun run lint, bun run format:check, bun run knip, bun run build.ts all green.