Add 'dispatch tags' to list tags with per-tag session counts

[jongio]

What

Adds a headless dispatch tags command that lists every tag in use with the
number of sessions that carry it.

dispatch tags
dispatch tags --json

Text output:

Dispatch tags

Tags:     3
Sessions: 3 tagged

  work      2
  personal  1
  urgent    1

Tags are ordered by count (then name), so your busiest buckets come first.

Why

You can tag sessions in the TUI and filter the list by a tag, but there was no
way to see the full set of tags you have used or how many sessions each covers.
Once you have more than a few tags it is hard to keep track of which exist,
which are used once and could be retired, and which are your busiest. There was
also no scriptable way to read tags from outside the TUI. --json closes that
gap and matches the shape of dispatch stats --json.

Behavior notes

• Counts are taken against the current session store, so tags left on sessions
  that no longer exist are not counted. This keeps the taxonomy honest.
• Unknown flags and positional arguments are rejected with a clear error.

Changes

• New cmd/dispatch/tags.go: runTags, parseTagsArgs, buildTagsReport, and text/JSON writers
• Command wired into handleArgs in cmd/dispatch/cli.go
• Help text updated in cmd/dispatch/main.go
• Shell completions (bash, zsh, fish, powershell) list the command
• README section added
• Tests in cmd/dispatch/tags_test.go cover report building, orphan filtering, text output, JSON output, argument parsing, loader errors, and the arg dispatch path

Testing

• go build ./...
• go test ./... -count=1
• go vet ./...
• golangci-lint run

All green.

Closes #256