docs: document the sourcing gate and the send skip report

[anusbutt]

Follow-up to #9 and #10, both of which merged before this landed. Documentation only — no code changes.

The README was describing behavior two changes out of date. Most notably, the send skip report from #9 shipped with no documentation at all, and it's the output most likely to be misread: already in ledger looks like an error when it's actually the duplicate-send guard doing its job.

What changed

• Capabilities — sourcing returns only companies not already in the vault; every stage explains what it left out.
• Flow diagram — now starts at source and shows the email-recovery step. Both were missing, so the diagram implied you always bring your own list and that a row without an address is simply lost.
• Safety guarantees — the "no company is silently dropped" row covered only run. It now covers all three stages: source reports already-known companies, run names anything it can't reach, send reports each skipped note with a reason.
• Send section — adds the skip report with a worked example and the common reasons.
• Review workflow — notes that an approved note is not necessarily a sendable one. That assumption is exactly what made the skip output confusing in practice.

Repo description

Updated alongside this (settings, not the tree) to mention the sourcing gate and that this is a personal project built in public:

Python CLI that researches local service businesses on the open web and drafts evidence-cited cold outreach into an Obsidian vault — then sends only what a human approves. Sourcing skips companies you already have. Email only; the offer is a swappable per-vertical profile. Never contacts Facebook. Personal project, built in public.

Verified

• pytest -q → 607 passed (unchanged; docs only).
• All README table-of-contents anchors resolve.
• Swept for stale claims — no remaining references to Messenger, fb_signal, prospector dm, or the deleted CONTRIBUTING.md.

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