feat(cli): add dispatch alias to set and remove session aliases

[jongio]

What

Adds dispatch alias, a CLI mutator to set, reassign, clear, or remove a session alias. Aliases were list-only from the CLI (dispatch aliases); until now the only way to set or clear one was in the TUI. This gives aliases the same CLI parity that tag and notes already have.

Forms

dispatch alias <id> review     # assign or reassign an alias
dispatch alias <id> --clear    # remove the alias on a session
dispatch alias --remove review # remove an alias by its name
dispatch alias <id> review --json

The <id> accepts the same short prefix that dispatch open does, and names reuse config's normalization and uniqueness rules, so dispatch open <alias> keeps resolving to exactly one session.

Changes

• cmd/dispatch/alias.go: runAlias, parseAliasArgs, resolveAliasSession, aliasResult
• Wired case "alias" into handleArgs
• Added alias to all four shell completion command lists (bash, zsh, fish, powershell)
• Usage line in main.go, example in the man page, ### Alias section in README, CHANGELOG entry
• cmd/dispatch/alias_test.go: table-driven parser tests plus runAlias coverage for set, reassign, clear, remove-by-name, duplicate rejection, unknown ID, and JSON output

Verify

• go build ./..., go vet ./..., go test ./... -count=1 all pass
• golangci-lint run reports 0 issues
• mage deadcode OK

Closes #331