docs: add CONFIGURATION.md + README pointers + community video link

Adds CONFIGURATION.md at repo root - a focused configuration reference
covering save paths, the per-source API-key matrix, reasoning and
web-search backend priority, the trend-monitoring stack (--store +
watchlist.py + briefing.py), and per-client patterns.

Surfaces two things that ship in the engine but were not documented
for users:

- The project-scoped .claude/last30days.env config file (currently only
  referenced in hooks/scripts/check-config.sh) which takes precedence
  over the global ~/.config/last30days/.env when present. Cleanest
  pattern for per-client setups - drop a file in the client folder, cd
  in, run normally.
- The existing trend-monitoring scripts (--store flag, watchlist.py,
  briefing.py) that the README did not surface for users.

Updates README with a brief "Configuration" section pointing to the new
file, plus a one-line "Going deeper" callout linking a 6-min community
architecture walkthrough on YouTube.

All CLI surface claims (watchlist subcommands, briefing modes, source
dedupe key, env file priority chain) fact-checked against the live
scripts/ source before commit.

README.md

@@ -280,6 +280,18 @@ skills/last30days/scripts/setup-keychain.sh --delete XAI_API_KEY
 
 Items are stored under service name `last30days-<KEY>` for the current user. On non-Darwin platforms the loader is a no-op, so there is no behaviour change for Linux/Windows users.
 
+See [CONFIGURATION.md](CONFIGURATION.md) for the full per-source key matrix, reasoning provider priority, and web-search backend priority.
+
+## Configuration
+
+Two things you'll likely want to know on day one:
+
+**Where research files are saved.** Default is `~/Documents/Last30Days/` (Windows: `C:\Users\<you>\Documents\Last30Days\`). Override with `LAST30DAYS_MEMORY_DIR=/path` in your shell, or `--save-dir <path>` per run. Use `--save-suffix=<name>` to keep multiple variations of the same topic separate (e.g. per client). Each run produces `<slug>-raw[-suffix].md`.
+
+**Trend monitoring across runs.** The default mode produces a fresh markdown snapshot per run. To accumulate findings over time, add `--store` to persist into a SQLite database, then use [`scripts/watchlist.py`](skills/last30days/scripts/watchlist.py) for scheduled runs (with optional Slack / webhook delivery on new findings) and [`scripts/briefing.py`](skills/last30days/scripts/briefing.py) for daily / weekly digests. The full cadence pattern is in [CONFIGURATION.md](CONFIGURATION.md#trend-monitoring-store--watchlist--briefings).
+
+Per-client wrapper scripts, custom category-peer subreddits, and the experimental beta channel for in-progress customizations are also documented in [CONFIGURATION.md](CONFIGURATION.md).
+
 ## How it works
 
 1. **You type a topic.** Person, company, product, technology, "X vs Y." Anything.
@@ -290,6 +302,8 @@ Items are stored under service name `last30days-<KEY>` for the current user. On
 6. **Synthesized into one brief.** Grounded in specific data. Cited by source. Ranked by what people actually engage with. Not "here's what I found." It's "here's what matters."
 7. **Then it becomes your expert.** After one run, your Claude session knows everything the community knows. Ask follow-up questions. Have it write prompts, draft emails, plan trips, architect systems - all grounded in what's real right now.
 
+> 📺 **Going deeper:** 6-min architecture walkthrough - [Real-time Social Listening across 10+ platforms: Under the Hood](https://youtu.be/SFejr5lJFxA).
+
 ## What people are saying
 
 > "I found a Claude Code skill that researches any topic across Reddit, X, YouTube, and HN from the last 30 days. Then writes the prompts for you. I've been manually searching Reddit and X for research before every piece of content I write. Tab by tab. Thread by thread. That's the part that takes 90 minutes. This eliminates it." -@itsjasonai