docs: sync config, env var, keybinding, and completion references

[jongio]

What

Fixes reference-documentation drift found by a full docs audit. No behavior changes: this updates help text, the man page, shell completions, the website reference pages, the README, and stale in-code citations so the docs match the actual code.

Why

A documentation audit found the reference surfaces had drifted from the code:

• The website Config page omitted ~10 real settings and claimed to list "all settings".
• Environment variables were documented inconsistently: no single surface listed all five.
• The README's remappable-action list was missing 6 actions users can actually rebind.
• docs/keybindings.md cited hardcoded keys.go/model.go line numbers that no longer matched (Ctrl+C cited line 65, actually 113).
• Shell completions couldn't tab-complete the existing notes and views commands.

How

• Website config.astro: added the 10 missing settings (ai_search, redact_preview_secrets, notify_on_waiting, attention_threshold, auto_refresh_seconds, default_collapsed, conversation_newest_first, default_sort_order, hidden_columns, keybindings) and dropped the "all settings" overclaim.
• Env vars, now consistent everywhere: added DISPATCH_CONFIG to dispatch help (main.go) and the man page (man.go), DISPATCH_SESSION_STATE to the README, and DISPATCH_NO_UPDATE_CHECK to cli.astro.
• README: completed the remappable-action list (all 63, including jump_top, jump_bottom, pivot_order, preview_fullscreen, tags, alias); added host to default_pivot, frecency to default_sort, and a redact_preview_secrets row.
• docs/keybindings.md: removed 96 stale (line N) source citations, keeping the stable file references.
• Shell completions (bash/zsh/fish in cli.go): added the existing notes and views commands so they tab-complete.
• config.go: doc comment now lists frecency as a valid default_sort.
• Consistency: standardized the product name to "Dispatch" in CONTRIBUTING/SECURITY; bumped CONTRIBUTING's Go version to 1.26.5+ to match go.mod.

All values were verified against the source of truth (keys.go, cli.go, config.go, model.go).

Testing

• go build ./cmd/dispatch/ passes
• go test ./... passes (full mage install and mage preflight)
• go vet ./... passes
• New tests added for new functionality (n/a: no behavior change)
• Existing tests still pass

Also verified: astro build succeeds (7 pages); the built binary's man and --help output now show DISPATCH_CONFIG; and all three shell completions include notes and views.

Screenshots

n/a: no UI changes (website changes are reference tables).