## Summary
Auto-discovers previously-unknown hashtag channels by scanning decoded
channel message text for `#name` mentions and surfacing them via
`GetChannels`.
Workflow (per the issue):
1. New channel message arrives on a known channel
2. Decoded text is scanned for `#hashtag` mentions
3. Any mention that doesn't match an existing channel is surfaced as a
discovered channel (`discovered: true`, `messageCount: 0`)
4. Future traffic on that channel will populate the entry once it has
its own packets
## Changes
- `cmd/server/discovered_channels.go` — new file.
`extractHashtagsFromText` parses `#name` mentions from free text,
deduped, order-preserving. Trailing punctuation is excluded by the
character class.
- `cmd/server/store.go` — `GetChannels` now scans CHAN packet text for
hashtags after building the primary channel map, and appends any unseen
hashtag mentions as discovered entries.
- `cmd/server/discovered_channels_test.go` — new tests covering parser
edge cases (single, multi, dedup, punctuation, none, bare `#`) and
end-to-end discovery via `GetChannels`.
## TDD
- Red: `34f1817` — stub returns `nil`, both new tests fail on assertion
(verified).
- Green: `d27b3ed` — real implementation, full `cmd/server` test suite
passes (21.7s).
## Notes
- Discovered channels carry `messageCount: 0` and `lastActivity` set to
the most recent mention's `firstSeen`, so they sort naturally alongside
real channels.
- Names are matched against existing entries by both `#name` and bare
`name` so a channel that already has decoded traffic isn't
double-listed.
- The existing `channelsCache` (15s) covers the new code path; no
separate invalidation needed since the source data (`byPayloadType[5]`)
drives both maps.
Fixes#688
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Co-authored-by: corescope-bot <bot@corescope.local>
Kpa-clawbot