feat(tools/discourse): forum bridge for projects that use Discourse instead of mailing lists

Add `tools/discourse/` to let the `issue-*` and `pr-management-*` skills read context from a Discourse forum where many modern OSS projects do design discussion, RFC posting, contributor introductions, and release announcements.

**Suggested capabilities:**

- `topic view ` — full topic + posts + tags + category
- `topic list --category --since ` — recent activity in a category
- `topic search ` — full-text search
- `post create --body-file ` — reply (write, gated on confirmation)
- `topic create --category --title --body` — new topic (write, gated on confirmation)
- `user ` — profile + recent activity (useful for `contributor-nomination` evidence)

**Why:** Discourse is the discussion platform for **Python core** (discuss.python.org — PEPs, ideas, help, packaging governance), **Rust** (users.rust-lang.org + internals.rust-lang.org), **Julia**, **NixOS**, **Elixir**, **Meta open-source**, **Docker**, **HashiCorp**, **PostgreSQL** (recently), **TensorFlow**, and several ASF podlings considering it as a mailing-list-replacement on-ramp. For projects that have moved discussion off mail, Discourse is the *primary* signal surface — without it the skills miss most of the team's actual conversation.

**Reference:**

- Discourse REST API: https://docs.discourse.org/
- The mail-source-contract abstract operations don't apply directly (forum ≠ mail), but the read-path output schema (thread id + author + body + timestamps) can mirror the mail-source shape for downstream-skill convenience.
- `contributor-nomination` is a particularly clear consumer — forum activity is part of the evidence set, but is not derivable from GitHub alone.