Add Grain to ~~conversation intelligence / ~~meeting transcription Included servers in sales and product-management plugins

Filing parallel PRs to add Grain ([grain.com](https://grain.com)) as a sibling Included server in the existing meeting-transcription category in two role plugins: `sales` and `product-management`. Both currently list Fireflies as the only Included server and Gong/Chorus/Otter.ai (sales) / Gong/Dovetail/Otter.ai (product-management) as Other options.

Both PRs are purely additive — a `grain` server block in `.mcp.json` (URL: `https://api.grain.com/_/mcp`) and `Grain` added to the "Included servers" column in `CONNECTORS.md` alongside Fireflies. This follows the existing multi-included-server pattern in both plugins (e.g., `Clay, ZoomInfo, Apollo` in sales's `~~data enrichment`; `Linear, Asana, monday.com, ClickUp, Atlassian` in PM's `~~project tracker`; `Amplitude, Pendo` in PM's `~~product analytics`). No skill, agent, or command changes. The existing `~~conversation intelligence` / `~~meeting transcription` placeholders in `call-prep/SKILL.md`, `call-summary/SKILL.md`, `competitive-intelligence/SKILL.md`, `synthesize-research/SKILL.md`, `stakeholder-update/SKILL.md`, `write-spec/SKILL.md`, and `competitive-brief/SKILL.md` all work as-is once Grain is wired.

**Why both at once:** Grain's user base spans both sales reps and product teams. Sales-team customers tend to compare-and-contrast revenue-intelligence tools (Gong / Chorus / Grain); product-team customers capture user-research and design-partner conversations in Grain natively rather than Gong. The two plugins are the natural home for Grain inside the role-plugin family.

**PRs:**
- #258 — `sales` plugin
- #259 — `product-management` plugin

**Related PRs in flight (filed alongside this issue):**
- #260 — `partner-built/brand-voice` (Tribe AI) adds Grain as a third meeting-intelligence source alongside Gong and Granola
- #261 — `productivity` adds new `~~conversation intelligence` connector category (Fireflies + Grain) and wires meeting transcripts into the memory bootstrap and update flows
- #262 — `partner-built/common-room` (Common Room) adds Grain as a sibling call-recording source alongside Gong

Happy to scope these together or split, whichever is easier to review. Other meeting-relevant role plugins (`customer-support`, `human-resources`, `design`, `enterprise-search`) could use a similar treatment in a follow-up wave — would prefer to land these two first to validate the shape.

**MCP details:** Grain MCP launched June 2025 — public, documented, used by Grain customers and Grain's own internal automation. Same SLA as Grain's main product. Reference: https://grain.com/release-note/06-18-2025 and https://developers.grain.com.

— Mike Adams, Founder/Chairman, Grain ([grain.com](https://grain.com))